Cuz the gram came before the SI system and the kilogram is a much more useable unit. The original m-g-s are based on physical things, like m being a subdivision of the length from the North Pole to the Equator going through Paris, and s being related to the time of a pendulum with certain length swinging or smth
A gram is the weight of 1 mL of water, roughly.
It’s called history, blame the French