Because the British drew them that way? I once read an essay that the British redrew lines in many places so that only a superpower could manage the region (of which Britain was a superpower at that time, relative to the rest of the world).

They're drawn that way because the western part of the Raj (now Pakistan) and the very eastern part of the Raj (now Bangladesh) were almost entirely Muslim, and the rest (now India) was almost entirely not. Once British withdrawal from the Raj became a fait accompli, the Muslim and Hindu populations were in conflict over the future of India. The Muslim League was agitating for independence for the majority-Muslim areas, and while the various groups explored the possibility of a federalist system that had some shared autonomy, that never gained much traction. By 1946-1947, the prospects of rising violence and imminent civil war were stark. Even though Gandhi opposed partition, Patel and Nehru came to realize that it was necessary to avoid massive bloodshed.

There was bloodshed anyway, but probably not on the scale that would have occurred if Partition had not happened (witness the atrocities in Bangladesh a few decades later).