Ubisoft wants so sell cheaper on Uplay to pull traffic there. Why not just simply dump Steam and ONLY sell on their platform? I'd really like you to answer this question.

Steam has major market share and it's not a good business decision to ignore that, so publishers are forced to submit to Valve's rules, unless they're ready to take the loss.

In a fair world, all these publishers would likely sell cheaper on their own platforms, which is good for you, the customer, but that won't happen with Valve setting rules, and so there is no reason for customers to make a switch, and so other platforms can't compete.

Epic gained some traction by giving away free games, but that's basically the best they can do.

Ask yourself - how can any other company be competitive in the current environment? How would you even go about building your own platform when the very basic thing (pricing) is basically regulated by the big guys?