That's the Doylist argument. The Watsonian one is that the people in movieland are all insane. Not only do they blithely walk into houses uninvited, but they also end phone calls by abruptly hanging up without warning, make arrangements to meet people without determining a time or place, they immediately throw away food they've just purchased because of a sudden change of plans, they hold weird extended conversations wherein two minutes of dialogue apparently occurs over potentially several hours, and the less said about their borderline psychopathic responses to death and violence the better.
Yes, all of these ticks are ultimately down to writing conventions, shortcuts and whathaveyous, but the end result is still a whole lot of deeply strange people.