So the brain processing power required wasn't that high, at least not in early FF games.
This is where we come back to the whole, people on average and such portion of the discussion.
You and I both read well. I mean, I think you read well. I've read your posts on and off on this forum for the better part of the last few years. Reading is almost a no brainer for us (I think, let me know if I'm wrong).
The world over, literacy rates are not 100%. And that's in each nation's native/common tongue.
Most video games don't get a translation into every language, and so in those places where literacy rates for a single language, not everyone can even read games that are reading intensive (if the game even sells in their country).
If you can't read what choice you're making, do you think you can strategize? Do you think you can even play the game?
Menu navigation and comprehension is a huge part of RPGs. People who don't read well, don't like opening and reading menus. It's an undesirable mental process, and an unwanted mental process is much harder for someone to deal with than a higher frequency of mostly subconscious ones.
Even most people who can read don't like reading. Reading speed and comprehension vary person to person. Someone may be 100% literate, but unable to read fast. To them, that single slow process of reading may take more brain power than positioning Mario half way on a pipe so they can get just enough of a running start to jump off that second gap-run floating block and make it across the pit.
tl;dr - Action games always come with the fact that you can perform the gameplay without a secondary skill/knowledge set that is independent of the game.
Make me play through Final Fantasy V in French, and I guarantee I won't be playing it as well as I can in English. Everything I do in it will be slower. I will make way more mistakes.