Back in the days of physical bookstores, it was not possible to offer every available book for sale. Some person at Barnes and Noble or Your Local Bookshop, would have to review which books they would have in the store, and which they would not. They would use a variety of factors, mostly how likely they thought it was the book would sell, but certainly they were free to use their judgement in whatever other way they liked to determine which books they offered for sale.

This curation was a part of the value the bookstore provided. Nobody ever demanded that they be forced to sell every single book in existence, or even that they explain why they did or didn't want to sell a particular book. Why should Amazon be any different?

If Joe Crackpot wants to sell his book online, he can make a website with a shopping cart and offer it for sale himself. He has no standing to demand Amazon's assistance in selling it.

Literally all intermediaries are making these decisions all the time. If I cook up some homemade ketchup in my bathtup, should I be able to force Giant to stock it on their shelves? Does Bloomingdale's have to stock their racks with whatever rags any aspiring fashionista sews together? Should Best Buy be forced to stock my homemade Fitbit that I soldered together in the garage?

No, of course not. They have people whose entire job it is to wade through all the available products and find the best ones to sell. There's nothing wrong with that.