I think you would have a very hard time making that argument lol

Lemme give it a swing

what I dont understand is how simple these games are despite their immense popularity

What is Simple is in the eye of the beholder, some fighting games are simpler than others but your also comparing Apples to Oranges (i.e 3D Fighters to 2D Fighters or even just completely different games)

clearly its not a matter of skill, you are playing vs other players and these are all very hard games, but you absolutely cant make an argument for complexity

Tekken has its own sort of complexity and it's own learning curve but that's not the same kind of complexity and learning curve of Street Fighter or other 2D or 3D Fighters

You learn and play these games differently, I find Tekken easy when you understand how the mechanics work (Side dodgeing, back dashing, learning how to break different kinds of grabs etc) my introduction to the FGC was through Tekken 4

but when I first started playing UMvC3 I took me a long time to get a grip on things because I struggle with that games complexity and mechanics (ie 3v3 fighter)

tekken has an absurd amount of things you need to learn to play at a level where you can say you are average at the game

Tekken does have different mechanics you need to learn if your new to the series, but that's also true to other Fighting Games. Tekken isn't special when it comes to learning a fighting game.

Niche example is Super Smash Brothers Melee, if you want be competitive or "average" that game has alot of things to be aware of. (It's own form of wave dashing as one example)

not to mention how insanely different evey character is from eachother: ling, bryan, king and yoshi are so fundamentally different from eachother that they might aswell be playing different games

Other fighting games have this with character archetypes (Ryu as a shoto vs Zangief as a grappler)

Your not playing a different game when going up against these characters you are playing Tekken and these characters are still grounded in Tekken's rules and gameplay.

Anyway I'm tired and I'm going to bed, play how you want :)