In real life, people do irrational things all the time. These things serve no purpose for anyone, at least no conscious purpose. They are thus truly irrational, and often highly destructive.
In a movie, if someone behaves irrationally, it seems to serve the story in a contrived way — like the writer/director has made this person do something they have no reason for doing in order to serve some preordained narrative end. And the audience won’t buy this — people in movies have to seem like they’re acting of their own free will, rather than fitting into a larger narrative scheme, even though, of course, that scheme (the movie’s plot) has no way of manifesting except through them.
So, I think, people in movies are not allowed to behave as irrationally as they do in real life because true irrationality is not achievable in a work of art that has an author.