And in most cases they accept blindly, otherwise there's no story. Madoka Magica is one of the few stories that I know where this trope is broken by the characters questioning the decision.
And not just American movies. Bud Spencer and Terrence Hill were super famous and did basically nothing but that. They don't train, never ever, they are lazy as cool factor and are strongest guys most sharpest shooters around. Franch movies, fairly often the same thing. Random middle aged man thrown in situation and suddenly climbing and jumping roofs.
And series targetted at kids still show plenty of training. Lego ninja go, winx club, all show training or classroom scenes often.
The point of his training is that the moves have completely become muscle memory/second nature. I have no clue how long that would take, but it would be significantly more than "low time investment".
Note also that (1) he claimed to have had karate training before; (2) after hebuilds up the needed muscle memory, training continues to teach him when to use what muscle memory.
However, it is kid/teenage fantasy. Which is fine! But not some kind of hard work message movie.
And half the plot is typically devoted to if they want to accept their new found powers or ignore them to live a normal life. So its still quite devoted to a choice with life changing consequences.
There is no way for Hulk to ignore or reject the powers. Nor for superman or spiderman for that matter. Superman in particular is just born that way, full stop. And speaking about action movies that nope, no big deep choices there. Even in matrix, the choice is singular scene where main character picks the pill so that movie can happen.
And I liked Bud Spencer and Terrence Hill movies, so I have seen them multiple times and consequences for choices and actions are not the theme there at all.
I have also read Spiderman comic targeted at kids back then. Half the time he just reacts to what is going on around, not having much choice nor choice being theme beyond "Am I going to do the right thing or not" false choice. Ninja Turtles had training in them ... but also they are essentially magical turtles due to some experiments or what done on them.
Average movie and entertainment in 90ties just was not that deep.
Or to use another example - gandalf might mentor frodo, but yet it always seems like frodo's accomplishments are his own and gandalf didn't really do all that much for him when all is said and done. Frodo is a hero and an interesting character, but i wouldn't want to be him.
Anyways i don't think the problem is lack of life lessons (even if they are lacking) its static characters and lack of stakes. Characters that were born ready are boring to watch. We like watching characters that change and adapt to circumstances whose choices are meaningful and consequential. The protagonist who was basically born with super powers, does not face a challenge and doesn't have to adapt their personality, is a boring character.