Because the police are not equipped to handle "petty" sexual assault cases as a routine. The only process they have to resolve these problems, besides enforcing a restaurant manager's request that a patron leave the building (which isn't a real problem), is to take a complaint and make an arrest, which is expensive not only for the victim but also for the accuser.

People have weird ideas about what the police do. Much as they'd like to pretend otherwise, they aren't referees or mediators.