Your control is greatly reduced in aerobars, but more importantly, your ability to defend your space is hugely reduced, which makes riding in a pack extremely dangerous. This video explains why having the ability to protect your bars prevents crashes: https://www.youtube.com/watch?v=sRIsWzrMmBs

Cycling has some dumb rules (e.g. sock length), but banning aerobars in mass start races is not among them.