The difference is probably that they don't have to interact with unknown factors (humans in cars).
City buses will be the last to go automated.
They could probably be significantly automated, but I don't know that the cost would be worth it, as even the best ones still interact with normal traffic at times.
Shouldn't that be automated first?
They have to slalom around badly parked cars, delivery vehicles, roadblocks, take alternative routes, &c. dozens of time per day
Many subway lines are automated. It just turns out that it is not cost effective to move between Level 2 (driverless, but the driver operates the doors and drives the train if needed) and Level 4 (fully unattended).
This is because the cost of the driver is less than the cost of having the system be fully autonomous. The Victoria Line in London, for instance, has been automatically operated since 1968. The driver just opens and closes the doors, and handles any unexpected situations.
Bit of a challenge to deal with them with software.
Not so for buses that serve rural communities. Where I live (admittedly an edge case) there are free-roaming ponies and cows that randomly create unpredictable obstructions and collision hazards.
However, I do think the handful of jobs do still make a difference. I recently visited a country which has conductors in each tram (so a driver and a separate conductor who goes around selling/checking the tickets while it drives). Everything else in the country is pretty much modern, always-online, and the majority of people pay with their phones - yet I still found it nice to see those simple, cheap jobs not be automated away. I'd much rather hold my phone to a thing and say thanks when that thing is a human, it's much nicer for me.
We lose touch to others so much already, why automate away perfectly efficient jobs? I haven't been in a bus that was late because the bus driver messed up.
TL;DR: I'd much rather have things done by humans as long as the machines replacing them aren't much better. Those are people who rely on those jobs to survive, and I don't see a point in replacing them.
I'd rather have universal basic income and a strong social safety net, so no one needs jobs to survive.
Automating away the jobs that people don't want to do is a step towards making that more economically feasible.