True, but that's a matter of distinction. If two regional variants of something aren't significantly different from one another, specifying the region doesn't make sense. It doesn't offer anything. There are plenty of foods which we don't assign origins to because they're generic such that no significant regional styles have developed, or they've become so "universal" that specifying a style is only done when called for, not in general.

Game genre's are similar - no, there is no distinctly JFighter or JPlatformer, but that's because there's no significant variation on the matter. Either there aren't enough entries to constitute a separate genre or subgenre, or there's simply not enough of a difference to separately categorize it. A whole genre will have multiple series from multiple producers, which the JRPG qualifies for. Mario, Castlevania, Metroid - these are just singular franchises, while the JRPG has a plethora of IPs, multiple similar touchstones, and style that is both distinct from western RPGs, and generally shared among most entries.

It's a legitimate distinction with a significant presence in the gaming landscape.

Buuuuuuut... as with all things, especially linguistic, tradition has outsized influence. We call things JRPGs because we've called them that for decades now, and will continue calling them that for as long as civilization stands. But... that's not a bad thing. It's a descriptive name that sets a reasonable expectation, same as any genre.