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.
To be slightly more polite, as this seems like one of your first places… You absolutely want to keep a neutral third party involved.
In my building, some of us are paying half or a third what new tenants do, the manager has a clear financial incentive to remove as many old timers as possible (and has tried her best.) If the norm was that building managers patrol for noise etc, you could much more easily get into a “he said/she said” with someone who has the means and motivation to remove you. Having the norm be police means that the complaint has to be somewhat valid, not just “enough that the building manager can increase their income stream.”
Landlord shit.