> However we'd have to see if that's truly the case, as cities have red lights and traffic, so the bus stops anyway.

Two problems - for one, riders entering and exiting takes time, especially if the public transit scheme says you can only enter at the front and have to show/buy tickets at the driver, and the other problem is that in most areas, buses cannot request a green light, so with a loop time of 1-2 minutes (quite common in German cities on busy roads) you may easily lose 2-3 minutes in the worst case just from a mismatch of departure with the light being green.

And over the course of a few stops, that lost time can add up quickly.