well, because the one we call alloy rims (aftermarket) aren't really 'alloy' hence i called them 'desi alloys' they are of no use -- they are brittle, hard and some of them cannot even stand a slight collision with a road divider.
stock rims are purpose made for the car -- there is whole engineering behind size, design and material of the rims used and manufactured by the company for its car -- leading to a softer ride and longer life of suspensions.