Isn't the whole point of fantasy not having to think too hard?
I kid. A bit. But it is true that fantasy doesn't tend to ask tough questions the way sci-fi does. You certainly don't want to think too hard about D&D settings because non of them make any sense.
The real world doesn't make sense half the time, doesn't mean we can't try to figure out how it works. I don't get the assertion that sci-fi is any more realistic. Sci-fi breaks all sorts of rules of physics and logic left and right. They just hide it behind pseudo science babble, but it's just different fluff to explain the impossible. Star Trek is chock full of BS.
What can I say? I want my campaign world to make sense. It makes more sense if I try to figure out consequences of magic and monsters being real. I'm not saying my way is better. But don't pretend that Warp Drive, humans having children with alien species, teleporters or being able to breath the same atmosphere and eat the same food is any more "realistic". It's not, and I say that as a fan of sci-fi.
Both science fiction and fantasy simply come up with explanations for the stories they want to tell. Some try to be more realistic than others, all fall short.