Ok but we're talking about countries not geographical regions. You do know there's a difference between the country called India and the Indian subcontinent, right? There are more countries on the subcontinent than just India.

Countries and their borders are political constructs. You are of course free to think that a country should have its border on this or that geographical feature like a river or a mountain range, but reality is often more messy than that.

Geography is just one of many considerations that factor into where borders between two countries ultimately end up, there's all sorts of political factors like history, demography, economic and strategic importance, etc.