The problem is some of those people won't want to stay in the hospital, and sometimes their habits are such that for regular hospital it'd be very bad match. And involuntary commitment is a very problematic proposition, it's a very dangerous tool esp. when used against people who can't explain themselves well, have no other people who can help them, and no knowledge or ability to help themselves.
To be more specific, US used to be more lax about involuntary commitment. And then it was discovered that all too often, people were committed for no good reason - sometimes due to negligence or pseudoscientific BS, sometimes deliberately.
Furthermore, once people were in the system, they were often abused themselves, and it was very difficult to get out, even if you were actually perfectly sane.
So committing someone involuntarily became much more difficult as a result.
