Because they’ve set up a false quandry. They’re setting it up as a three option problem with no other options.
a. Use a Windows desktop to game because they prefer mouse and keyboard unless Steam comes out with a desktop. b. Use a Steam Deck with mouse and keyboard but it has alleged poor support for that and isn’t viable (I’m saying alleged because I’ve never used a Steam Deck though my son has one). c. Use SteamOS and hope it has better keyboard and mouse support than the OS on the Steam Deck.
Those aren’t your only options. You could also.
d. Use pretty much any other Linux based distribution on a desktop and install the Steam app on it. Why would you need a Steam based OS? Clearly they don’t have a problem running Steam as an app in Windows, why not do the same on Linux?