But, yeah, there are exceptions and corner cases to everything. I believe the fundamental idea is that "real" life didn't used to be like that, nor is it today. You just don't see the stuff you see online happening at the corner store. And the difference is that it's people you might see again, or can call the police on, or just recognise as fellow humans.
The problem of anonymity sometimes being required also isn't entirely new. There have long been guarantees (or attempts at such) of anonymity for situations where the need for it was generally acknowledged. In one such case, the organisation even adopted it as their name: Alcoholics Anonymous.
The people doing the harrasment do so under their own names.
people would avoid getting mental help, support groups, religion questions, sexual health, etc if they worried it would be easy to be exposed.