And anyway, buses and trains don't even have seat belts.
Beyond that, it's probably a matter of practicality that passengers aren't required to be belted in. It would significantly reduce the capacity of public transit. A better option might be to protect the driver from projectile humans. AFAICT that's exactly how modern buses are designed.
I don't know if this applies to general passenger buses or just school buses, but in the US they have closely spaced seats with energy-absorbing seat backs that limit the force on each passenger and how far the passenger can move in a collision.
Smaller school buses (under 10k pounds) are required to have lap and/or shoulder belts at all seating positions.