Funnily enough, the Rivian CEO talks about exactly that in this interview

In that scenario, we would be using one ECU to do everything I just described. In this case, it’s a much larger computer, but one computer. It’s a massive simplification for how we think about software development and also drives a lot of cost out of the vehicle because instead of 70 to 80 little boxes — little computer boxes with wiring and connectors and everything else — we have, in our case, seven.

So when one small thing stops working, you now have to buy their proprietary, expensive ECU instead of a cheap little microcontroller.