I never really consciously thought about it before, but I figure it's your speed. Longer banks with actual surface will cause more friction and thus gradually slow you down as you're trying to go against gravity, whereas on a bump there is little to no time for friction, you literally just hit the angle and launch as most of your speed is redirected and converted into upwards momentum, not killed. Sort of similar to the functioning of wallies vs. wallrides. That's probably part of why I could skate even just a sidewalk bump all day to be honest, it's just some endless fun trying to see how far (and in which ways) you can optimize your approach of them to get this or that kind of boost on your tricks. Also photos of people getting onto absurdly tall stuff thanks to the help of what sometimes really is microscopic terrain detail are always inherently fascinating.