It's because car manufacturers are loath to change microcontrollers in their vehicles because they've got decades of processes, tooling, and debugging with the (Atmel) chips they've been using since forever. When they decide to make a new car they basically just look at the latest Atmega(whatever) "automotive" chip (using really old chip tech) and choose that.

Atmel has "automotive" chips for everything! From regular MCUs to beefy ones with boatloads of pins and (slow ass) LCD controllers. They've made it so that car manufacturers don't even have to think! The engineers probably get an automatic OK to use whatever Atmel "automotive" chip they want but anything else requires a lengthy and expensive certification process.

Some cars are using STM32 chips made for automotive but they're not as common as you'd think!

Basically, the car manufacturers are extremely risk-averse because of low margins and something like an ECU recall can totally ruin the profitability of a new car. They're also lazy and don't want to try new things! There, I said it 😁