The car screen is significantly bigger than the phone screen, making it quicker to glance at it for driving instructions.
But now we're just coming back to Apple CarPlay and Android Auto. I just want a big screen with physically touchable controls for those. My previous car did exactly that, but now I've gone near two decades older so I now get a fancy screen with no functionality beyond FM radio and DVD video lol
For legal reasons it might be easier to take an existing car, throw out all the tech, and add your own. You won't own the chassis design, but you can at least use open source software everywhere.
Difference between getting a modification certified, vs a self build.