Involuntary in-patient psych care is different than prison because it is focused on improving someone's life, managing a potentially dangerous condition, etc. They (should be) a hopeful place, a place of healing. They don't always live up to that, of course, but at least the objectives are correct.

Prisons are not places of rehabilitation in the US, and a person who is suffering from a severe mental illness is likely to suffer profound abuse in prison.