That really depends upon what the "chips" the carmakers are using do. If they are using highly integrated micro-controllers similar to these (this is but one e

That really depends upon what the "chips" the carmakers are using do. If they are using highly integrated micro-controllers similar to these (this is but one example of this family) https://ww1.microchip.com/downloads/en/DeviceDoc/40300C.pdf https://ww1.microchip.com/downloads/en/DeviceDoc/40300C.pdf then to "emulate" this chip with an ARM CPU involves also designing external hardware to interface with the ARM chip to recreate all the built in interfaces of this one chip (i.e., the analog comparators or the PWM controller or the serial UART). Other chips in this family have an even larger complement of integrated interface hardware. Changing to a new CPU does not always involve just writing new software, esp. in device controllers it also involves recreating all the hardware necessary to interface that new CPU with the existing hardware that the old chip was controlling. And recreating all that interface hardware requires a redesign and a retest of that redesigned hardware. Both of which can take significant time.