Similar to how pilots often ascend from freighters to passenger aircraft, self-driving systems should have started driving freight through the desert. (Or mining pits, for that matter.)

Passenger operation as well as operation around people should have come much later, and step by step. Error rates must be determined step by step, and reliability ratings must be earned, similar to ETOPS.

I can only see one reason why they would begin with taxi operations, the driving task with the most visible showing of servitude, and that reason has to do with US history.