They are not able learn on chip - that is a non-starter and not particularly useful anyway. Customers dont want self-driving cars that need to learn how to drive, they want self-driving cars that already know how to drive.
1. Learning does not have to happen inside a car. But it does have to happen somewhere, and that's where the learning in hardware will be much more efficient/faster than learning on a GPU.
2. There are scenarios where local learning would be necessary/preferable to remote learning (e.g. one shot learning or continuous online learning).