3) The Fitness requirements !
The body requirements of a bowler and a batsman are very different. The exercises they go through and even the physical problems they face are very different.
It is impossible to expect a Kohli like fitness from a bowler. Although this cannot be an explanation but it definitely is a reason. People rightfully prefer to dedicate their time into a single domain and excel there.
How it hurts the country as a whole is a different story. Another reason does Team India not have an all rounder.