Why don't people move into the carriage?

It still amazes me that in the mornings when I get on the train, there seems to be this common theme of people getting onto the train and not moving away from the door so more people get on.

I mean really, if there's people behind you, what do you think those people are going to do? Be happy to be stuck on the platform?

Why don't people move into the carriage? I've often had no patience when seeing someone get on and just stand there, I just push them. Is it that people are selfish and don't care about other people getting on the train? Or are they just so stupid to realise that other people exist?

I really don't know.