A 6 cm baseline is enough for humans to make adequate distance estimates.

Besides the correspondence problem, a longer baseline makes it hard to keep the cameras aligned as the vehicle bounces and flexes. You can't mount them separately to the car -- a chassis can easily twist by a degree or two. So you need a stiff mounting bar between them, which you can either put outside the car like a roof mount (ugly, and it gets buffeted by wind) or inside (also ugly).