¿Why are there so many countries whose names ends in -stan?

Some examples are Afghanistan, Kazakhstan or Pakistan

The suffix -stan is an anglicised version of the Persian for ‘place of’. It is connected linguistically to the Pashto -tun and to –sthāna in Indo-Aryan languages.

These derive from Proto-Indo-Iranian-European roots based on ‘stā’ meaning ‘to stand; where one stands’.

Its widespread use may be a result of commonly developing languages of various communities of nomadic people across central Asian areas over time.

The same root is also the source of the Latin ‘stare’ (to stand) and from there to English words such as stand, state and status.

Other derivatives are the Russian word стан (stan) referring to settlements/camps of semi-nomadic people of Central Asia; some Slavic languages where stan originally meant ‘settlement’ but more recently has come to mean 'apartment'.

The root can be found in various Germanic languages for exemple in Stand and Stadt (German), stad (Dutch/Scandinavian), Stan (Polish) and stead (English; as in ‘homestead’).

The –stan suffix often simply meant ‘land of the …..’. So Uzbekistan = land of the Uzbecki people; Afghanistan = land of the Afghani people; and so on.

Pakistan does not follow this construction. The name Pakistan is not derived as the land of some (historical) ‘Paki’ people but means Land of the Pure. The difference is because Pakistan is a new, and invented, name to describe a politically-defined area and not a historical word for the traditional homelands of a single long-established cultural/ethnic group of people. I

In a number of languages the –stan ending is also used more generally within everyday words: as in the Urdu rigestan (a place of sand ie desert), as in golestan (a place of roses ie rose garden); as in qabristan, (a place of graves ie cemetery or graveyard); and as in the Hindi/Sanskrit devasthan (place of devas ie temple).

In English there are seven ‘obvious’ recognised countries whose names end in –stan. These are: Afghanistan, Kazakhstan, Kyrgyzstan, Pakistan, Tajikistan, Turkmenistan, Uzbekistan.

The same linguistic root also shows through in the names for different countries in other languages. For example Arabestan (Persian for Saudi Arabia), Armanestan (Persian for Armenia), Bulgaristan (Turkish for Bulgaria), Chinastan (Armenian for China), Ermenistan (Turkish for Armenia), Hunastan (Armenian for Greece), Hayastan ( Armenia in Armenian), Gurjistan (Georgia in Persian and Turkish), Lehastan/ Lehestan (Armenian and Persian for Poland; and the older Ottoman use referring to the Polish empire ; derived from name of ancient Lęch tribes), Macaristan (Hungary in Turkish), Parsqastan (Iran in Armenian), Rusastan (Armenian for Russia), Vrastan (Armenian for Greece) and Yunanistan (Turkish for Greece), Engelestan (Persian for England). Hirvatistan (the Turkish name for Croatia) and Sırbistan (the Turkish name for Serbia).

These are far from fixed names. There are older usages that have become obsolete. Language is an evolving thing and this is as true of names of places as for other changes in language usage. Some regions are regarded as ‘independent’ by some groups but not by others.

What counts as a country can be a complicated question. For a good summary the reader is referred to the Economist article ‘In quite a state: How many countries in the world?’ (www.economist.com/node/15868439). This describes how on one set of criteria a place may be included as a country on some lists yet be excluded on other lists (because not officially recognised by the owners of that list).

Many of the regions whose names end in –stan are in areas that are diverse and multi-ethnic with boundaries that are the result of historical events. The boundaries, name, or existence of some –stans may therefore be matters for disagreements.

Even for disputed territories, however, the linguistics can still apply: Chechenestan is the Persian and Turkish name for Chechenya. South Ossetia is another self-proclaimed state which has varying degrees of formal recognition. Iriston/Iristan (from aryi+stan) is a self-proclaimed name of Ossetia.

Regions/ towns whose names end in –stan (an extensive but probably incomplete list; includes some descriptions that might be disputed by groups seeking independence of / or opposed to independence from certain historical arrangements).

Arabistan — refers to Arabian peninsular lands in Middle East; was also historically used in some reports to refer to Khuzestan

Ardestan — a town, founded in ancient Sassanian times, in Isfahan Province, Iran.

Avaristan — the Avari name for homeland in Western Dagestan (fromC12th to C19th).

Baharestan - is an area in downtown Tehran where the Iranian Parliament is located.

Balawaristan — (balawar = highlander); another name for northern Pakistani Kashmir; alternative name for Gilgit- Baltistan).

Balochistan/ Baluchistan — regions in Iran, in Afghanistan and in Pakistan. Baltistan — a mountainous northern region in Kashmir Pakistan.

Bantustan — used to refer to Apartheid-era South African black 'homeland' areas. Bargustan/Borgustan — an area to the north of modern Kislovodsk, Russia. Bashkortostan (Bashkiria) — a constituent republic of Russia. Baloristan (Gilgit-Chitral) - the name of a region of Pakistani Kashmir.

Cholistan Desert — a desert region in Punjab, Pakistan. 
Dagestan —(literally "place of mountains") an ethnically-diverse, North Caucasian, constituent republic of the Russian Federation.

Dardistan — ‘area inhabited by the Dards’; is a region spreading over northern Pakistan, Indian Punjab and North Eastern Afghanistan.

Dashtestan — a region in Bushehr Province, Iran. East Pakistan (or Bangalistan / Bangistan - refers to the historic name for preindependence Bangladesh).

Frangistan/ Frengistan/ Frankistan - a central Asian term used to refer to Western Europe in general (Based on Europeans being known as Franks).

Gulistan/Golestan - a province in northern Iran and a city in Uzbekistan. Hazarastan/ Hazaristan - the homeland of the Hazara people in central highlands of Afghanistan.

Hindustan — (land of the Indus/ Hindus). Coined by the ancient Persians. Also used by the British ruling in the former British India when generally talking about South Asia. Now primarily refers Republic of India.

Hunistan — ‘kingdom of the Huns’; in Semnon Province, Iran.

Kabulistan — (‘The Kabul land’). An old term used in many historical books and old Persian literature books for an area around Kabul, larger region than today's Kabul Province.

Kafiristan — (‘land of the infidels’). An historic region in Afghanistan until 1896, now known as Nuristan. A similarly named region exists in north Pakistan.

Karakalpakstan — an autonomous republic of Uzbekistan. Khuzestan — a province of south-western Iran. Kohistan — there are several districts with this name in Afghanistan, in Pakistan, in Tajikistan and in Iran. Kurdistan — a Kurdish region spanning Eastern Turkey, Northern Iraq, North western Iran and Northern Syria. Lazistan — a name for a region in the Caucasus; home of the Lazuri speaking people. Has been part of a series of occupations and empires. In 1922 the area was split between the then Soviet Union and Turkey. Lorestan/ Luristan/ Larestan — a province of Iran. Moghulistan (Mughalistan) — an historical geographic unit in Central Asia that included parts of modern-day Kazakhstan, Kyrgyzstan, and Xinjiang. Nuristan Province — Afghanistan; formerly was an area that was known as Kafiristan (land of the infidels) but changed its name to Nuristan (land of light) when area converted to Islam.

Pashtunistan or Pakhtunistan or Pathanistan — what many Pashtun nationalists call the Pashtun-dominated areas of Afghanistan and Pakistan. Registan - (meaning "place of sand") a UNESCO World Heritage Site in central Samarkand, Uzbekistan. This large open space was a public gathering area between three madrassas.

Sakastan — historically, a region of Afghanistan/ Pakistan where the Scythians or Sakas lived in the 2nd century BC.

Sarvestan — a town in Fars Province, Iran. Seistan or Sistan — a border province between North Eastern Iran and South Western Afghanistan.

Tabaristan — an historical region along the southern coast of the Caspian Sea. Takestan/ Takistan — a town in Qazvin Province, Iran.

Talyshistan — an ethnolinguistic region in the SE Caucasus and NW Iran.

Tangestan — a region in Bushehr Province, Iran. Tatarstan — a constituent republic in the Volga District of the Russian Federation.

Tocharistan, Tukharistan or Tokharistan, also known as Balkh or Bactria — the ancient name of a historical region in Central Asia, located between the range of the Hindu Kush and the Amu Darya (Oxus). Turkistan/Turkestan — an ethnolinguistic region encompassing Central Asia, northwest China, parts of the Caucasus and Asia Minor; Russian Turkestan refers to that portion of Turkestan that was in the Russian Empire, later becoming Turkestan Autonomous Soviet Socialist Republic. There are also Afghan Turkistan and Chinese Turkistan, Xinjang area). The city of Turkestan is in present-day Kazakhstan.

Waziristan (North and South) —semi-autonomous regions of northwest Pakistan. Zabulistan — an historical region in the border area of today's Iran and Afghanistan, around the city Zabol.

Zanjistan, or Zenjistan — a term used in medieval texts to refer to the homeland of the Zanj, ie black slaves of East African origin, ie area around Zanzibar. 
Reference:
http://thewordsthething.org.uk/Countries%20ending%20in%20Stan.pdf