I don't think the issue with learning fighters is difficulty
It's sort of something I gradually arrived at after years of trying to get into it, by seeing how and when exactly I kept dropping fighting games, and when and how I actually kept playing for a while.
And I think the answer is actually super-simple: The "learning" in fighting games is rather decoupled from "playing (in the truest sense)".
Examples from other genres... when I want to learn to speedrun Emerald Hill Zone Act 1 in Sonic 2, I play Emerald Hill Zone Act 1 in Sonic 2, a lot. When I want to learn to be better at Civilization V, I play a campaign of Civilization V. If I want to improve my Pokémon team, I have to take it into ranked matchmaking to see whether it works.
In all those games, the way to improve is to directly play. And in all those games, I probably fail badly early on at those goals - but either the games are slow enough (Pokémon is turn-based, as is Civilization V), or the play error feedback is obvious enough (you can see clearly what went wrong in a Sonic speedrun), or the amount of variables is low enough (like Sonic 2's simple controls, even in contrast to the intricate physics), that I improve simply by playing.
In fighting games, the play error feedback and the way you internalize things don't really function quite the way they do in most other game genres, in my experience.
It can be very hard to focus on inputting your combos and specials correctly while you're getting bodied, because the controls are inherently more finicky than in other genres of the same level of play speed and inputs per second; it can be very hard to understand what you did wrong in the thick of it, because the gameplay is simultaneously very technical but also continuous (as opposed to being turn-based).
Case in point, look at the other genre that's seen as notoriously hard to get into, RTS - replace "difficult controls" with "micromanagement" in the above paragraph, and you basically have the same thing going on.
So how you learn fighting games is often training mode and reading guides - you step away from playing to learn instead, and "instead" is doing all of the explanatory lifting here; it's always the point where I drop out.
The fighters I played the longest, meanwhile, exemplify this from the opposite side - BBCF and Pokkén. In other words, the game with the big and flavorful tutorial mode and the extensive story mode, and the game that also has a whole story mode and also lacks motion inputs and lets me focus more on other play aspects, respectively - letting me get further without requiring me to set aside "playing" to do "learning" in training mode and guides.
Ultimately I think this really underlines the importance of singleplayer content, and for it to be extensive and deep enough, as a way to immediately start having fun while learning and improving.
Fighters are damn cool games, anyone with eyes in their skull can see the sheer potential for amazing fun when observing how skilled players do their thing - that isn't and never was the issue or remotely close to it for any new player that doesn't have a baseline aversion against the style of gameplay. It's just the decoupling of "fun play" and "learning".
... or maybe I'm full of crap, who knows! Thanks for reading.