Too expensive compared to what?

Any kinds of transit system requires dedicated infrastructure. Humans in particular need signs, road markings, traffic lights, etc.

Decades of research and optimization went into building that infrastructure so that it would be easy for humans to use safely and effectively.

Why is it surprising that this infrastructure simply doesn't work well for computers to operate in?

We aren't trying to run trains on unmodified streets. We build rails first. We should do the same for self-driving cars.