1) The ‘India’ thing !
Well this is the most fundamental and scientific reason anyone can think of. When you are young and playing with a group of kids in your locality, you cannot bat first if you bowled first and vice versa.
So even if you are a combination of Mitchell Starc and Virat Kohli, you will end up doing just one of the two things.
Science says that you could develop the basic skills best during early years. Not so much in India man. Another reason does Team India not have an all rounder.