Nobody is forced to use their service. Your point that untenured professors must "publish or perish" doesn't fully analyze the situation. Tenure committees have no intrinsic reason to favor these publishers. They care only to the extent that these publishers have the brand that signals to them and others: "hey, this author doesn't completely suck."
Now, the publishers largely inherited these brands, from back when publishing was a more involved process involving paper. But there is nothing nefarious keeping them in the system. Any open access publisher could rise up tomorrow and as long as they could develop the brand and signaling power they could take on the publishers. But regardless of anything else, branding, sorting, filtering, and signaling are things people want, even more than free and easy access to information. And the publishers stay in the loop because they offer those things.