There's something irritating about people mocking the realism of fiction, in an attempt to allow anything and everything.

As though the constraints of good storytelling are nothing at all...

"Zombie Dinosaurs in Space" is an absurd idea... but it can be done well, in a realistic fashion, or it can be done poorly in a unrealistic fashion.

Realism is the manner in which a story is crafted so as to quell your disbelief.

A fantasy with magic can be realistic, as long as their are rules to explain how the world works. Once you know the rules, you can begin to believe the world is possible... despite it not being so

When you say "that's not realistic" about a fictional story, what you're really saying is "I can't believe that would happen in this world."

You can't reconcile the thing you're commenting on with the greater whole, and so the greater whole becomes unrealistic.

In this regard it's not the realism of our own world that we're interested in, but the realism of the world that we're pouring our time, interest & passion into.

And for that world to be a worthy vessel for those things, it needs to be of sound, logical construction.

Consistency in regards to cause and effect. Binding rules that can't be broken, even to favor protagonists or the narrative itself.

These are the things that make a story realistic, no matter how fantastical. Break these things, & even the most mundane story becomes unrealistic

People who mock, or dismiss, the realism of a narrative, typically in pursuit of having it changed to suit themselves, are really saying they don't actually care about the story... they just want their desires met, regardless of the damage it does to the work as a whole.

Realism and canon are integral, because they're the load-bearing structure of a story. Without them, the story isn't a worthy vessel for peoples time, interest or passion.