Fair point, Kate, Arkham isn’t designed to make mentally ill people better. Even if individual doctors and such might go there with the intention of helping the patients… Gotham being Gotham, most of the time it’s meant to operate as a “specialist“ prison for Gotham’s costumed criminals.
And since Arkham is subject to the same underfunding that causes the GCPD to be so notoriously corrupt, there are a TON of references to the mistreatment the patients are subjected to over the years.
Such as in Birds of Prey #60, where a guard freely admits to selling medical information and photos of patients showering to tabloids.
Or even in Batman: the Animated Series, where the villain Lock-Up enjoyed physically abusing the less physically dangerous rogues (Scarecrow, the Ventriloquist, Harley Quinn) while he worked as a guard in Arkham, justifying his treatment because they’re “criminals“. Notably he didn’t try this stuff with the more physically dangerous patients, such as the Joker, Poison Ivy, Two-Face etc. And when he was called out on his behaviour, he claimed that “the liberal media“, among others, were to blame for his not being allowed to just brutalise vulnerable people whenever he wanted.
The thing is, that there are other mental health facilities avaliable (Selina Kyle’s sister Maggie ended up in a nice one in Catwoman’s preboots series, for one example, Tom King’s upcoming superhero-centric Sanctuary facility).
It’s just that people get sent there either because, presumably, the other options are too expensive (a teenage pre-supervillain Black Manta was sent there for “treatment” of his autism), spite (a whitecollar criminal was sent there by a judge out of spite because he stole the judge’s pension), or because the writers flat out forgot other options existed (Ray Palmer commiting Jean Loring there for her murder of Sue Dibny).