Why does no one ever move back on the bus?

Something that's been driving me bonkers lately is how people are so terrible at moving back on a crowded bus. So many times I've gotten on a 14th Street bus and there's a huge crush of people in the front but nobody in the back. 

Usually some of those folks can clearly see there are people who are trying to move back and they don't move, just staring blankly. Then somebody has to yell "can you move back please" and people look around and scoot a little. Then more people get on and the painful process repeats, with diminishing returns as people seem to get tired of being asked to move, even when there's still room.

It's not rocket science, people. If you see some space behind you, move back into it -- or if you're next to the one doofus who is blocking everyone's path, ask them politely to move back. Everyone else ahead of you will appreciate it.