Because it isn't my actual JOB, that's why. That's their job, that they applied for, and we PAY them to do. Nobody is forcing them to be cops. How are we supposed to trust them to clean up crime in our city, when they can't even clean it up in their own workplace?
If someone in my house was an active criminal, I WOULD do something about it, but it's not up to me to do anything about anyone else. That's called vigilante justice, and it's illegal. We hire COPS to do that. Then they won't, because it's too dangerous.