People here are missing that developers aren't benevolently intent on driving down prices. especially not in a way that differentiates between where you buy their product. It would also cause massive issues with storefronts because it directly drives sales away. It'd create insane friction if they did this in a way that wasn't at least playing within a certain set of agreed upon clauses.
This stuff is why EA, UBI and all the others started their own launchers in the first place. To avoid legal issues from discriminating storefronts and eating more of the pie. They don't sell their games cheaper on their own platform, they just take more pie.