It would be nice if it was that easy. Unfortunately, it's far too easy for HR to give the boss a pass, especially when the accuser is below them (and considered less important to the company).

And it's not too difficult for that boss to find ways to get you fired if they feel betrayed by you reporting them. Giving you impossible deadlines, tasks you aren't well suited for, dropping hints to other people that you're not performing well, etc. Corporate politics are ugly and are designed to get around HR.

Or, maybe HR takes your side, who knows, but I think a lot of people are afraid to report out of fear for their jobs.