If the argument is about live-action adaptations of animated shows/movies that have no print source material, such as Cowboy Bebop or Avatar, then I would agree that they may not be adding anything new that wasn't or couldn't be explored in the original medium. But I also don't think that makes live-action inherently bad. A faithful adaptation isn't automatically a bad one, or even necessarily a pointless one. Sometimes the only point is to entertain you.
But my question was more broad, since the mood of this thread seems to be that any live-action adaptation of anime is going to be bad, because that's just how it works. But I'm not really seeing an innate difference between live-action adaptations of American comic books and live-action adaptations of Japanese comic books, and why only that first group can have good movies.
I think one of the issues when it comes to translating animation to live action is how characterization is told through animation. Animating single frames is expensive and time consuming, so animation makes the most out of key frames that convey expressions and body language, this is especially the case for anime which is infamous for limited animation and holding poses for long periods of time. The physicality of this doesn't always translate to live action; compare Jet to Spike, whereas Jet is Netflix Bebop is almost universally praised for being accurate to the anime, Spike's portrayal is more divisive. There's a lot of reasons for this but one of the biggest I think is Jet was very much the "straight man" of the Bebop crew, so his physical expression and gestures were much less exaggerated compared to someone like Spike, who had so much characterization conveyed through the way he walks, poses, fights, etc. And attempts to draw on the more exaggerated nature of physical acting in anime can come across as campy.
Now, you can tell by my avatar, I am a fan of Kamen Rider. I am no stronger to campy live action effects-driven shows. But those shows are also designed from the ground up with those limitations in mind.
As for the differences between adapting American comic books and manga, I think one of the biggest is American superhero comics don't really have a linear narrative. There's smatterings of books across decades with wildly different writers, stories, and tones. Characters like Batman and Spider-Man have had such a wide variety of portrayals that adaptations can draw from multiple sources, mix and match, and feel like their own thing while staying true to the source material. Most ongoing manga is more serialized in general, with a clearly defined narrative. This means adaptations tend to stick closer to the manga's story beats and narrative, which will constantly draw comparisons. Manga adaptations in Japan are also usually done mostly to promote the brand and bring up merchandise sales, which is why most will stick to anime, because it's cheaper and easier to translate to animation. Live action adaptations of manga tend to be rather slapdash and done just to promote the brand, like Fullmetal Alchemist and JoJo. There are exceptions I'm sure, I've heard the Kenshin movie is good (which makes sense as a straightforward samurai story would be much easier to adapt), but very rarely do these studios put people in charge who want to tell a hand crafted story, but instead often just something to promote brand synergy.
tl;dr, manga adaptations tend to stick to anime and that's where the quality is, most live action manga adaptations are hastily made to make some easy money.
