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.
Not sure where you live, but that hasn’t been my experience at all.
There could be regional variations based on things like tenant laws, local law enforcement policies, and even local culture.