If you're in an urban environment at least as densely packed as, say, San Jose, then the bus system is likely to have actually quite substantial ridership. By the time you get to the kinds of dense urban environments of the standard major US cities (NYC, Chicago, DC, Philly, Boston), I quite assure that the buses are routinely packed. The story might be different if we're talking about, say, Bismarck.
Furthermore, I should point out that if a bus is stopped "pil[ing] up traffic behind them", that's because it's letting passengers embark or disembark, which is a strong signal that the bus probably isn't 90% empty.
You're falling victim to the inspection paradox: https://towardsdatascience.com/the-inspection-paradox-is-eve...
In practice, this means someone pockets the change and nothing gets done about the so called externalities.
Citing this extra tax, some lawmaker spends money on something completely irrelevant that was lobbied to them. Public transport is a fairly distant concern for most of them.
Oil companies increase crude prices to get on the gravy train, so does every other entity in the industry.
Car dealerships jack up prices because they want to maintain their profit levels.
Meanwhile middle class people who rely on cars get effed.
On the rare occasion that the “public transit” busses sharing our roadways are full, they are indeed very efficient.
Unfortunately - these situations are rare. The rest of the time, other solutions are vastly more desirable to the public. Simultaneously, these huge, lumbering 90%-empty beasts pile up traffic behind them, forcing massive exhaust plumes from the unnecessarily idling traffic.
Stop subsidizing sub-standard solutions, and let the transit-using public decide.