Because self driving cars aren't entirely self driving.
Particularly in Tesla's case, the 'autopilot' mode is basically an upgraded cruise control. It's not *really* meant for you do let it do its thing, it's there to help, but you're still nominally in control, and being responsible for a multi-ton metal box hurtling along at 40+ mph, you probably should keep an eye on it, so you can intervene and take over if something goes wrong.
AIs aren't really magical, they're just statistical models applied at high speed. There's no real decision making involved, it's just 'look at the camera, best guess for what these pixels are is...' based on mathematical similarity to other pictures.