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.
I’d be willing to bet that nearly 100% of apartment leases have some language in them about noise meaning apartment managers absolutely do have the power to enforce these limits. Even HOAs, where you buy the property with your own money can enforce their own rules because you agreed to that when you signed the paperwork.