Because the problem with the former is that people with mental health issues will likely not cooperate.
And the problem with the latter is that forced medical treatment because someone's mind doesn't work the way you think it should is a very very dark thing to start doing...
She is a schizophrenic woman in Boston iirc. Watching her delusions and obsession with various objects is fascinating and heartbreaking. I really want to talk to her just to try to understand better. I think it is impossible simply because something is wrong inside her brain. I wouldn't be able to rationalize why traffic cones aren't some kind of grand conspiracy of the Patriarchy. I think she is homeless now, I hope she gets the help she desperately needs.
At that point it can be hard to force them to have treatment against their will. This is for good reason - forced psychiatric treatment has been abused before. Forcing someone to have treatment against their will is an interference with their human rights. We tolerate that if they pose a risk of serious harm to other people[1] or to themselves.
Another reason we don't just scoop homeless people off the street and forcibly medicate them is that treatment for psychosis isn't particuarly great - it's not effective for some people and the medications carry a range of severe side effects.
The best thing we can do is provide correctly funded "assertive outreach teams". Here's a description of one assertive outreach worker's day: https://www.theguardian.com/healthcare-network/views-from-th...