I think the answer is - because they’re lazy and want you to do their dirty work for them. I quite frankly, am not going to call the police over noise complaints because I think management should do something about that. Police should only be called when violence or tenants who get aggressive.
Not because of noise, I just think it’s management dumping responsibility onto you when they’re the ones with the power to evict people.
Building manager isn’t your parents. If you have a problem with the building, that’s their issue, if you have a problem with your neighbour, you’re expected to deal with it like any other adult; talk to the person, if that doesn’t work, police.