Why is Europe considered a separate continent while India is considered part of Asia?

Europe and Asia have been together for much longer than India and Asia and the boundary between Europe and Asia is less substantial than the boundary between India and Asia.

The accepted boundaries of Europe and Asia came about for cultural reasons, not essentially geographic. It began with the ancient Greeks who recognised a boundary between their cultures and others in Asia Minor.

As the geography of the region became more understood, the arbitrary division of Europe and Asia extended along cultural lines, between societies viewed as 'Asian' or 'European'. Some of the boundaries follow geographic features such as mountain ranges and lakes, but that's because it also it marked tribal boundaries.

The accepted boundaries were established before the advent of modern geology and the understanding of plate tectonics.

The two continents collided most recently during the Oligocene, so the present Eurasia postdates India's connection with Asia.

it seems you dont know geography very well

Europe and Asia can be united in Eurasia

India is a unique sub-continent