When the place treating someone is a prison, safety, security and punishment necessarily take priority over recovery and care. So sending mentally ill people there for treatment guarantees the treatment loses whenever the two goals clash.
Harsh confinement does not just fail to help people who are already mentally ill; it routinely makes them worse. It also makes mentally stable people psychiatrically unwell. The system takes away a person's sense of self and soundness of mind along with their freedom.
Policies that treat symptoms of mental illness as crimes move large numbers of people out of places meant for healing and into places meant for punishment. Once there, they meet force, isolation or neglect where they needed care and a safe place.
Nearly everyone who goes to prison is eventually released back into the community. If confinement leaves them more broken than when they entered, that harm returns to everyone's streets, so cruel imprisonment ends up punishing the public too.
Practical reform means probation and parole systems that give people real resources to rebuild their lives after release. It also means regular mental health checks for every prisoner, so that illness is caught and treated instead of left to worsen out of sight.