Because that's pre-crime law, and it's more dangerous than the problem it's trying to fix.
I do not think that removing dangerously mentally ill people from the streets is more dangerous than waiting until they act out on their violent tendencies.
Once upon a time, mentally ill people could be confined to hospitals indefinitely if their illness warranted. Which pretty much means they stayed for life, since true mental illnesses cannot be cured. "Civil rights" groups have successfully stopped the practice of institutionalization, and the results are not good.
It isn't just the dangerously mentally ill people who were thrown out on the streets to prey on others, but non-dangerous ones, those we now call the "homeless." The homeless cannot take care of themselves; throwing them out on the street is just plain cruel. But society does not really suffer when the homeless are thrown out to fend for themselves, not the way it does when the psychotic mentally ill are thrown out.
Whenever there is a highly publicized case of a mass murder, it seems that the murderer was giving clues all over the place that he was about to do something. And no one acted to stop the murders. In today's reality, it is nearly impossible to do anything--the mentally ill, dangerous or otherwise--cannot be institutionalized. And once the psychotic ones commit a crime, then they end up in the prison system, which really is not set up to handle the mentally ill.