Microcontrollers are used to reduce weight. Instead of a thick bundle of wires going to the door locks and power windows and power mirrors and door open switch and any lights that happen to be there - you have a couple of data and power wires going into the microcontroller unit that controls all the door stuff.
This goes for everything electronic, and there is a *lot* of this stuff in modern cars. Tail lights, rear climate and entertainment controls, radar parking aids, tire pressure monitors, heated seats, cabin lights, cabin temperature sensors, microphones, etc... Overall weight savings are in the dozens of pounds.
This is all done to drop weight to make CAFE standards. You could standardize on a different microcontroller, but these things are purpose-built and a full environmental TA soak can take years. You don't want one of these things to fail and have to tear doors apart to replace them in a recall.