I somewhat agree with Sajam but the lack of guidance and pedagogy is the biggest issue in fighting games that does make them significantly harder.
This is a popular Tekken meme, beginners does that often but that's because they have absolutely no guidance. Combos are the only thing they know through osmosis, and they have not even a single idea of what is before combos. They do not even know it exist even if they watch tournaments. Maybe some will understand these but still don't know how to approach it. A beginner could do a korean backdash but not know how to make it useful. They know the movement because they saw it, heard it, find it cool, but they don't know that movement is about keeping a distance and creating situations that are beneficial to you through whiff punishes, poking or other things.
Mobas have stronger osmosis even if they don't have a good tutorial system. They get to understand how the system plays. They need to do a last hit on creeps, they need to earn money, they need to keep their distance. They will get their ass kicked, but I definitely think there is a good learning experience from matches that you will not get in a fighting game to the same extent.
They understand things easily because the tutorial system is the game itself and they can see people in their team doing things they can replicate step by step. The game design promotes a learning curve that is interesting. Playing randoms online in fighting games will not deliver this same amount of learning because it is more intricate. Someone jumping on you constantly will not make people connect their brain cells that they have to use anti-airs. They don't have replays to see what they did wrong, even if they have replays they don't have a structure that makes them understand where their failings are. Fundamentally, I think you do not really learn from your mistakes unless you have a great mindset from the get-go, but this is not the fault of the person itself. It's like people claiming to be bad at maths, it's because they haven't been taught to have a mindset to break problems in smaller problems. They don't even know that there are steps to break them down in the first place. They see weird algebra shit and they give up. I know I did.
There is this lack of pedagogy that makes them harder than they could be even if fighting games are not as hard as you think it is. I admit I don't have a learning attitude, I like to spend time in practice mode and I have learned it doesn't offer me as much benefits as I would have liked than just fighting people. I do learn things and I think there are things that I would have learned much easier if there was some kind of pedagogy associated with the game that is so far removed from making the game easier through easier inputs or the like. It's about something more than simplifying a game.
It's about guiding the player to a direction that promotes a learning environment that isn't present in as many fighting games as you would expect. It's a huge part of game design that should not be ignored. To just teach a player that a fighting game is about learning situations and creating new situations is such a game changer that can change the whole outlook of a player's philosophy towards fighting games and make them better despite not doing anything. Just that can change everything.
I think it is missing and it is something that even video guides will not teach if it's not built into the game itself. Remove the aimlessness that they feel, promote players to have a focus, a goal to meet that is not just about combos but about the entire philosophy of a fighting game and I'm entirely sure it can push beginners to go as far as they want to be. Shooters can deliver that, MOBAs can deliver that. Fighting games can deliver that far easier than these other genres can provide. There is a ceiling that can only be broken when fighting games will be able to teach a mindset.