Valve is not the one setting the prices. The publishers/developers are. One could argue that they are increasing their prices becauses of Valve’s cut, but they aren’t. A Ubisoft game for example costs the same on Steam as it does on the Ubisoft store, which is obviously not taking any cuts for its own games.
Also, 30% is the industry standard. Here is a nice overview IGN made in 2019. https://oyster.ignimgs.com/wordpress/stg.ign.com/2019/09/GameRetailerCuts_infographic-1.png
Edit for the price parity argument: If the parity would have increased game prices to include Valve’s cut, AAA games would have gotten 30% more expensive many years ago. But they didn’t.
Yea, until devs started leaking the dates way in advance for some attention. I prefer official communication over the dev of “Washcloth Simulator” spoiling the surprise.