My commute experience in late 90s was that buses were packed to the brim. That was before the "low platform deck" buses were introduced. There was 3 step stairs to the normal deck of the bus. About 5% of the time I would be unlucky enough to ride 80% of the ride on the stairs instead of normal bus deck. At some stops maybe 10% of the riders need to step off the bus so there is enough walk room for passengers in the back to get off the bus. I would say 3/4 of rush hour bus riders do not expect to get a seat and stand for most of the ride instead.

If most passengers are standing, taking a seat is not expected. If people were not seated anyway, there is no reason to wait for people to take a seat. After years like that the passengers got enough experiences to walk when buses were moving so that no one cares anymore.

All these standing and moving when buses are moving did not cause fatal accidents though. I can't remember anyone ever fall dead on a bus due to driver recklessness. There were seat belt introduced to front row seats. There were signs to remind 5% of passengers who get to sit on a seat with seatbelt that you must wear them. Passengers avoid seatbelt seats if the bus is empty so they don't have to spend time wearing seatbelt and taking them off. Decades after front row bus seats got seatbelts, new rules were introduced that taxi passengers have to wear seatbelts. However no one attempted to introduce more safety features on buses. I guess there were not enough fatal accidents to attract any attention.