Why do you have to wear a seat belt in the car but not when on a train?
It's very unlikely you'll be in a train crash where a seatbelt would have helped.
The train operates on tracks. The potential for accidents is much smaller. Also in a train the seat belt would do nothing.
Why don't you have to wear a seat belt on a bus?
You do here though, if the bus has them. Although I have never seen anything but private buses have them.
Even so, nobody wears them.
I believe that bus drivers use a seat belt.
because it's funny to see those inside cameras when buses crash and everyone flies around bouncing off the walls
Busses in public transport don't go very fast and stop regularly. Also, at least here, they have special privilegs in traffic (you have to let them pull out of the stop for example) and often their own lanes.
You do have to wear seatbelts in coaches if they have them .
Seat belt laws are intended to make you accustomed to following directions even when using your own property and acting "independently". They exist to remind you that you are not in control (of anything) and that your purpose is to serve and fear the state at all times.
The reason you don't have to wear one on a public bus is to make you associate the government bus with freedom. Anytime you want, you can move around and just get up and press the stop button.
Practicality. City buses aren't meant to operate anywhere other than on low speed streets. They disintegrate in crashes. If anything, not being strapped to it is a feature, in a crash you might get lucky and be thrown clear.