Everything on the one PCB is great, just ask Tesla owners that get stranded with a hardware failure, or need to reboot their screen for a software failure by holding down steering wheel buttons to get their car operable again.

The reason cars have lots of computers is for price - its cheaper to use a power and CAN data bus rather than have individual wires going from a central computer to each device.

For example, a door module can control the locks, windows and mirrors with just a 2 power wires and 2 thin wires for CAN bus.

Contrast this to old cars which had thick bundles of wires to power the mirror, doorlock and window motors from outside the door.