Anime's pretty popular. You can hate on the otakus all you want, but it's a good medium, and creators who are somewhat familiar with the medium past the stereotypes know this. It then, doesn't come as a surprise that studios want to exploit it for making live action adaptations.
Hollywood's been at it for a while, I think we can all remember the disaster that was Dragonball Evolution (no magic genie dragon can undo the mistake that was) and just how bad it was in every conceivable way. The plot was a terribly, terribly watered down adaptation from the first series, and the cast all had the acting range of a teaspoon.
It doesn't help that they also look nothing alike: anime character designs are awful. No amount of gel on a white boy would get you Goku's hair, ever. And no amount of real people appearance would look anything even remotely similar. The CGI doesn't help either, and makes it extremely immersion-breaking.
More recently, Netflix's Death Note was poised to be another attempt at a Hollywood cash grab from a mainstream manga adaptation. Now, Death Note compared to DBZ, is far more grounded and 'realistic', with no insane character designs minus Ryuk, who ironically would end up being the best looking part of the film. Death Note was still widely panned (all comments about whitewashing aside, that argument alone merits its own article) because it was a lacklustre adaptation that turned an engaging battle of minds to a meandering emo high school mess with a convoluted plot.
So what actually keeps going wrong in all these adaptations?
The inherent problem is the medium itself. Anime works because it's so unnatural; no one really acts like normal people because most anime is inherently so defiant from reality's rules. Mainstream shonen anime like DBZ, Naruto, etc. all run on otherworldly rules with exaggerated antics and actions that replicating them into real life just makes you cringe and shudder so live action just feels wrong to begin with.
Now you can point your fingers at Marvel/DC/superhero films and any Hollywood film with fantasy elements and go like "but they're making money". You're not wrong, but it's for good reason. Superhero movies haven't been panel by panel adaptations of any arc. Thanks to the inherent story structure of comics, it's much easier to pick and choose loose elements from many years of stories and translate it better to screen. They're also much simpler and easier to explain: Batman's origin story and how Gotham thrives is easier to explain to the average person than say, Luffy's and the world of One Piece.
Anime usually rests on long running continuous arcs, and often relies on good exposition and episodic/chapter pacing to tell its story. The new live-action Fullmetal Alchemist is a good example of this mistake, a vast story with good world-building, all forced into a two hour narrative that greatly diminishes its impact.
Loose adaptations didn't seem to work out well either, even with a grounded story in Death Note. What went wrong there?
Bad writing, bad writing in general alone breaks a story, no matter how perfectly well set up it was for a change in medium. But even in the right hands, the adaptation still needs tweaking. Giving creative freedom to make a newer, or a different story is the best way to go, but for series' that rely on a 'fixed' story, it's just offending old fans, as well as making a shallow product for newcomers, just like what Netflix did to Death Note.
Some anime can work however, but they're the ones too mundane and less flashy for Hollywood to try. Guillermo del Toro once wanted a TV adaptation of Monster, which might've been a Game of Thrones level hit. Oldboy (the South Korean one) and Edge of Tomorrow are actually live action adaptations based on manga that did work great. The Japanese making their own ones are also a lot better, Rurouni Kenshin, Beck and Soranin are all terrific adaptations. You can definitely pin a lot of bad creative choices on Hollywood's part and a critical misunderstanding of the medium as reasons for bad adaptations.
But ultimately, anime doesn't lend itself well to being made live action. Even with a good creative team behind it, making changes is necessary, and relying on flashy blockbuster properties laden with CGI only makes the unrealism glaring. Making changes to the story and loose adaptations are necessary, but that's all easier said than done. Making TV shows instead of movies might work well, as most anime does lend itself better to longer formats. But ultimately it's easier to do things on paper than executing those ideas, and perhaps without revising their ideas of why anime works the way it does and is popular, Hollywood execs might just be best leaving it alone entirely.