[bobobooey]

Due to a lot of the controls and even speedometer only being accessible through the screen, it's probably easier for them to keep it on their own OS.

[Gaelan]

Lots of cars display their own UI chrome around CarPlay.

[shawnz]

Presumably the Tesla software contains a lot more custom built functionality than just a customized UI

[Gaelan]

I think CarPlay runs basically entirely on the phone and the car manufacturers just add a feature to their existing software to show the pixels coming from the phone.