The dearth of mental health institutions is not (directly) because “elements of our polity” don’t want to pay for them. It’s a legacy of actively closing them in the 70’s-80’s due to inhumane conditions and civil libertarian concerns around institutionalizing people against their will.
Now - we certainly aren’t having a very productive public conversations about the obvious negative outcomes of that policy shift.
No, of course that is a large part of the story, but things like the Willowbrook expose also caused left-wing politicians (in that case Robert Kennedy) to also attack institutions.
It's a result of not funding appropriate community-based supports after deinstitutionalization. Deinstiutionalization could have worked out very, very differently, if we'd been willing to pay for the alternatives.
Now - we certainly aren’t having a very productive public conversations about the obvious negative outcomes of that policy shift.