While by no means exclusive to either Eastern (namely Japanese) or Western (namely American and European) fiction, this trope is almost a given in Japanese media. Indeed, it's actually rarer to find Aversions of this trope than instances of it being played straight. In Western works, particularly Comic Books, it's played far straighter.
This page hopes to offer at least one explanation as to why.
Japan, being a collectivist society, places a higher priority on proper methodology and work ethic than the west. Shonen characters and superheroes, while similar on many levels, are fundamentally different. Shonen heroes are usually throwbacks to old Samurai concepts, like Bushido, as well as Buddhist and Shinto values. The combination of these ideals typically creates stories about a hero who has to raise themselves in some type of rank or hierarchy through hard work and training. Shonen characters place a greater value on creating techniques or fighting moves with their powers, which is essentially a way of creating something to train for. In short, training is essential to the usual Shonen story structure.
In the west, however, superheroes are basically extensions of a capitalist/freedom-based culture. Rather than being focused on conforming, American superheroes are focused on the idea that superpowers grant characters some level of autonomy or individuality. Superman, the Trope Namer, has an identical power set to his fellow Kryptonians, but rarely do any of them create forms, techniques or moves with these abilities. Sure, every individual power has a name (such as Heat Vision), but that's about it.
It's possible that this stems from the old Western value of "Grace"
—that is, gifts or blessings bestowed by God upon people simply because God Is Good, not because those people particularly deserve it. As we said before, Eastern culture tends to be based on a hierarchy, and thus that hierarchy is enforced/earned in some manner. Western culture, on the other hand, usually considers hierarchy a "bad" thing; for instance, Batman may be a less powerful character than Superman, but stories will often go out of their way to demonstrate that he is still no less "equal". Compare this to Japanese stories, where even after going through several Next Tier Power Ups, most characters still adhere to a Fixed Relative Strength.
Using "Grace" as a theory for this trope, it stands to reason that Western media thus treats superpowers as a "gift" from some sort of higher power and thus training is unnecessary. After all, it would be considered bad form if Moses looked up to God and said, "Thanks, Lord, but the ability to part the sea isn't enough. I'm gonna go train to make these powers even better."
