Cost is much higher than for the commoditized chips they use. Instead of developing high end SoCs that could run multiple systems, they went for these cheap microcontrollers that controlled a single function or just a few.

To give you an idea of what we are talking about, here’s Infineon’s product page. They are a big automaker supplier, and right in the middle of all this. https://www.infineon.com/cms/en/product/

Not if you want automobile rated durable chips with higher withstanding voltages etc. On many MCUs, the CPU itself is a minority with a large part being 5V tolerant IO etc.