There also isn't a Matrix, Time Travel isn't real, there are no space wizards, There's no buried pirate ships is Astoria, etc. etc.
I'll suspend disbelief if the story is good. The only time I get pissy about realism is if the movie presents itself as serious/realistic like Ad Astra otherwise if you want Time Traveling Cyborgs or whatever.. i'm down as long as it's fun.
But I agree most people probably aren't aware of something like John Carter of Mars because people today only know toys and video game mascots. What can you do.
What I'm saying is, if it takes place in a setting that people are already familiar with, but things are different than they know them to be, that requires at least a minimal amount of explanation. On the other hand, if the story takes place in a wholly-invented fantasy world, then little to no explanation is needed.
The Land of Oz - why are things like that? They just are, it's a place you've never heard of before the story and that's what it's like.
Star Wars - same thing, they specify right at the start "A long time ago, in a galaxy far, far away..."
But if you're gonna set the story in "the real world" that people know, but things are very different, you need a reason. It's the future. Aliens opened a dimensional gateway. Someone invented a time machine and now everything is all screwy. Whatever it may be, but it requires at least a basic story explanation.
The big difference is that when the John Carter stories were originally written, Mars was totally fair game for whatever. We knew it existed, it was a planet, and... that's about it. Some basic telescope photos but all pretty blurry, and not much else. It could be ANYTHING on that planet, so go nuts with the story.
When the John Carter movie came out we're already well into the Mars Rover program. The general public knows it's NOT like what is depicted in the movie and the vast majority of people have never heard of John Carter of Mars before, don't know it's based on books published quite a long time ago, and so on. They don't try to explain away the disconnect, and that's a problem.
There are other problems too; he can jump really far because of his Earth-strength... higher gravity, fair enough, but that doesn't seem to translate into any other form of strength or power for some reason...