Why should he stay behind? What does he owe to the people of the state of India? If already gave them far more than they would get from most of the poor people.
It is not about owing anything to the people of India. It is about owing himself the pleasure and satisfaction of making his homeland - the place where he was born and the place where he grew up - a better place.
He grew up on Earth. He is making it a better place. Why settle with just improving India? Why not the world.
Simple: because India is a developing country? It needs all the help it can get?