The police are "nothing" - they are the default handlers of antisocial behavior, but in no way are they mental heath specialists.
wrt mental health centers: what happens when people don't want to go to those places? Either they'll be in exactly the same position of being handled by the police, or the police will be too busy in which case the public will suffer (which usually changes the electorate opinion on matters).
This is what people talk about when they say defund the police. They want to downsize police to the point where they only deal with crime and use the freed up funds to run mental health centers, halfway houses, drug treatment centers, etc...