It's also because they want to advance gaming on Linux specifically.

When Windows 8 released, Microsoft was pushing their app store, ARM devices that could only access the Windows store, and Windows Phones. Valve became scared - Microsoft clearly wanted their future to be installing software through the Microsoft store.

They also added an "Xbox" app to windows - a clear indication that MS wanted to bring that business to PC, and have an unfair advantage by having their services pre-installed.

Gabe Newell has worked for Microsoft before and he knows just how ruthless and anti-competitive Microsoft can be. He knew that while Valve was 100% dependent on Windows, Valve was at risk.

So they brought out the Linux client and released Steam Machines, which as we all remember were a flop due to limited game compatibility and poor performance.

So Valve got to work on adapting WINE to create Proton, a Windows compatibility layer integrated into Steam. They put money into Linux development, then, when Proton was good enough, they released the Steam Deck.

Make no mistake, the Steam Deck is ultimately a part of Valve's overarching plan to reduce dependency on a hostile competitor that controls the entire platform that they operate on.