One interesting thing about it is that it's so closely associated with, how shall I put it, poverty and misery. It doesn't really matter how realistic a romantic comedy which takes place among the upper class is, it will never be discussed in terms of realism. But if it's a story about depraved immigrants or a struggling working class mother, it will almost by default be praised as realism, or at least realistic. One might wonder why actually. Is it because film critics (and people in general) are rather gloomy and thinks that "real" life sucks, and that it's unrealistic to be rich and happy? If you compare a glossy romcom like Four Weddings and a Funeral (1994) and any film by, say Ken Loach, is there really any difference in actual realism, by which I mean, is there anything in Four Weddings and a Funeral that could not happen in real life?
Isn't the term "realism" more than anything else an ideological statement? Either used consciously to state a preference for a certain kind of film (regardless of whether or not it's actually more "real" than any other film), or used unconsciously because you think it's the culturally acceptable thing to like.
Another thing about realism is that it's sometimes confused with explicitness, or with showing everything. It's often used as a reason (or excuse) for showing graphic violence or graphic sex scenes. It has nothing to do with realism though. It isn't more realistic to show something than not to show it. If person A is slaughtered with a sledge hammer, it might be shown in depth, with blood everywhere, very realistically. But it might also be done with person B raising the sledge hammer over person A, and then there'll be a cut, and in the next scene we see person C saying "Person A has been killed." That isn't less realistic. It's just less blood.
There's a parallel here to lying. Very often being completely honest is equated with telling everything. But it's not the same thing. If I ask you "How are you feeling?" you might answer "I'm feeling very bad." because you are feeling very bad. So you're telling the truth. The reason why you're feeling bad might be that your cat just died but you don't have to say that in order to be honest. Only if I ask you "Why are you feeling bad?" do you have to tell me about the cat. Or, you could also answer "I don't want to say." and if you don't want to talk about it, that would also be true. The film The Invention of Lying (2009) has made this misconception the basis of the whole film, and it's a flaw. If you tell people things which you haven't been asked about, the issue isn't that you're honest, the issue is that you always tell people what you're thinking all the time. That has nothing to do with honesty. If I say "Hello" to you, it's not more honest of you to answer"You are extremely ugly." instead of just saying "Hello.", even if you think I am extremely ugly.
And so it is with realism. Showing everything isn't more realistic than showing nothing, or part of something. Again, realism isn't what you show or how you show it, it's the characters, situations and motivations.