Is you car running a "modern" operating system ? Probably not (i hope not).
The operating system is only a miniscule part of the overall AVRs cost, which is quite price sensitive due to the ongoing competition between manufacturers .
AVRs usually run operating systems optimized for their tasks and resources. They usually have much less processing power (CPUs) and memory than a typical smart phone. They are sort of embedded plattforms, many probably using specialized microprocessors or maybe ARM based kernels with Linux derivatives known for their low footprint and bandwidth needs or something similar.
I don't know, if anyone uses Windows Embedded Compact (Win CE).
An visually oriented all around platform like Android is much more power hungry for things, which are of no use in an AVR (except maybe the setup menus).
The realtime processing power needed is in the DSPs used, which typically have their own operating environment.