Since the end of British colonialism in the Indian subcontinent, the countries of South Asia have been largely divided on an ethnic basis. The largest and most populous country in the region, India, is home to over 1.2 billion people, with the majority of its citizens being of Hindu ethnicity. Pakistan, on the other hand, is a majority Muslim country, with a population of around 190 million. Bangladesh is also a majority Muslim country, although its population is much smaller, at around 160 million. 

The other countries of South Asia are Nepal, Bhutan, and Sri Lanka. Nepal is a majority Hindu country, with a population of around 28 million. Bhutan is a Buddhist country, with a population of around 740,000. Sri Lanka is a majority Buddhist country, with a population of around 20 million. 

It is often said that South Asian countries are not divided on an ethnic basis. This is not strictly true. There are, in fact, many ethnic groups in South Asia. However, these ethnic groups are not evenly distributed across the region. 

The largest ethnic group in South Asia is the Indo-Aryan group. This group makes up about 60% of the population of the region. The second largest ethnic group is the Dravidian group. This group makes up about 20% of the population. The remaining 20% of the population is made up of various other ethnic groups. The distribution of these ethnic groups is not even. The Indo-Aryan group is concentrated in the north and the west of the region. The Dravidian group is concentrated in the south. The other ethnic groups are found in all parts of the region.

The reason for this uneven distribution is historical. The Indo-Aryan group is descended from the people who migrated to South Asia from Central Asia. The Dravidian group is descended from the people who migrated to South Asia from the Mediterranean region. The other ethnic groups are descended from the people who have lived in South Asia for many centuries. 

The uneven distribution of the ethnic groups has led to some conflict. The most serious conflict has been between the Indo-Aryan group and the Dravidian group. This conflict has been going on for many centuries. It has been responsible for the partition of India and Pakistan. The conflict between the Indo-Aryan group and the Dravidian group is not the only conflict in South Asia. There are also conflicts between the different ethnic groups. These conflicts are not as serious as the conflict between the Indo-Aryan group and the Dravidian group. 

The reason why South Asian countries are not divided on an ethnic basis is that the different ethnic groups are not evenly distributed across the region. 

So why are South Asian countries not divided on an ethnic basis? 

There are a number of reasons. 

First, the religious and cultural diversity of the region means that there is no one dominant ethnicity. 

Second, the countries of the region have been through a long history of migration and settlement, which has resulted in a mixed population. 

Third, the political borders of the region were drawn up by the British colonialists, and do not reflect the ethnic reality on the ground. 

Fourth, and perhaps most importantly, the countries of South Asia are united by a shared history and culture. Despite the religious and ethnic diversity of the region, the people of South Asia share a common heritage that is based on the shared experience of colonialism and independence. 

This shared history and culture gives the people of South Asia a sense of common identity that transcends the religious and ethnic divisions of the region. It is this sense of common identity that has allowed the countries of South Asia to remain united, despite the many challenges that they face.