An observation I've made comparing heroes from both sides of the Pacific is that in Anime/Manga, most heroes train constantly to gain new powers and become stronger, because they aim to become the strongest/beat their rival/whatever.
American superheroes don't seem to care about becoming stronger, they're usually content with their power levels. After being defeated, they don't go "I must become stronger to beat X" they'll just go "I must beat X", they don't seem to have any ambition or drive in this regard. We'll occasionally see them training in the Danger Room or whatever, but we don't really see the results of said training in action leading to new abilities. We get more screentime of heroes eating than we do of them training.
It's true that over the years some heroes seem to become more powerful, but that's solely due to writers not caring about previous stories and doing power creep, which is often ignored by subsequent writers anyway. There are no real "training arcs".

Why do you all think this is?