I think there is a danger of being overly pompous about this. People
push in because they can and because they gain advantage from doing so.
In a car they also feel protected from negative comments and thus are
less inhibited by the danger of social exclusion.
I think one of the reasons many motorists are uncomfortable with
cyclists is because cyclists are much more likely (than other motorists)
to talk to them and criticise them for their anti-social behaviour.