Android auto/Apple CarPlay is screen mirroring. The apps run on your phone, not the car.

The benefit is that all the app and OS updates your phone receives are reflected in the Android auto experience, without needing the vehicle manufacturer to constantly ship updates and provide support for the head unit of your car for years after it’s originally manufactured.

Using a cellular connection for that would be very slow, laggy and unreliable. It would also use a ton of data, which someone would have to pay for.

A number of reasons:

• uses a lot of data (car data plans are often pretty low)
• relatively slow compared to 5GHz
• coverage, you ideally want your navigation and music to keep playing even if you go out of range