They might have as well moved to Windows handheld or Nintendo Switch. They specifically chose the only Linux handheld on the market.

No, they chose a Steam console. A device with the same high convenience and low bar of entry as any other console, but with their (almost) whole Steam library on it.

Why is this a core requirement with Linux only? There are millions and millions of Windows users who have never installed an OS. Sounds gatekeeping to me.

Because conciously choosing and installing Linux is currently the requirement to run Linux on your PC.

If I go to the local electronics store I can pick up a Windows, MacOS or ChromeOS device that has everything pre-installed: OS, drivers, dependencies, all setup for instant usage.

And if I don't even know what an OS is, I'll get a Windows PC recommended by the sales people at said electronics store.

That kind of user experience is usually not available for prospective Linux users.

Unless they buy a Steam Deck, which is pretty much the only native Linux PC that's popular enough that a non-tech person would know it.

(Technically stuff like Tuxedo and Framework exist, but they are pretty unknown.)