Valve is doing this for strategic reasons and also because they wanted to start the handheld PC market (Steam Deck). Strategic reasons: Microsoft could at any point buy several important gaming studios or distributors, distribute a lot of games (maybe exclusively) via their own store (they even announced that several years ago, but they didn’t do it in the end). MS could even implement small things which make Steam perform worse on Windows, as its 100% controlled by MS. If you compete directly with Microsoft on the Windows platform, you will eventually lose because MS can do some very tiny tweaks which happen to make your product be more annoying or slower to use than Microsoft’s own. That way they’ll still fly under the radar for anti-competitive behavior. So Valve has to ensure that their main business model (selling/distributing games on Steam) remains future-proof, and that means more independent from Microsoft’s agenda. To do this, they need to push a fully neutral, but viable alternative to Windows for gaming. Which is Linux.