I can't speak for everyone, but there is a spot in my city where I will briefly get up on the sidewalk with my bike, and for me the reason is always the same, the street isn't safe. The downtown where I work has no bike lanes, no shoulder, and people blatantly running red lights, blowing stop signs across 4-6 lanes of traffic while speeding. There is no greater joy than getting doored because the other alternative is swerving further into the road and getting run over at 40+ mph. For those two blocks it's safer for me to slowly push the bike along with my feet until I get to the next block and ride in the street again. Too many cyclists that get on the sidewalks don't show deference to people walking, the same way they wished cars would show some deference to them.

It really depends. IMO you should ride wherever is safest for everyone. I would say more people use scooters on the sidewalk than bikes. And the 15-20mph is dangerous claims don’t hold up when you can literally go 1mph or walk it if the sidewalks have pedestrians. Going 20mph on a sidewalk with people around is dangerous. Going 20mph on a sidewalk that is empty when the road has shit or no bike lanes is most likely much safer for everyone.