What stops authors doing that is that most authors struggle to sell 50 copies at far lower price point no matter how much effort they put in because most books just aren't interesting enough to enough people to be easy enough to sell without it turning into gruntwork paying far below minimum wage.

Congratulations, you're a massive success as an author, having sold more than most authors will. That's amazing.

But consider that most authors have no interests in writing business / self help books, and most who do still fail spectacularly because having success with those kinds of books tends to happen primarily if you already have an audience and a track record in the field you're writing about. E.g. the perceived value of your book is 99% your story about your track record, and most would-be authors don't have that track record.