Because the time to learn a new operating system is more than I have to spare. I'm approaching middle aged, windows is familiar and I just want to kick back and enjoy my games.
Linux hasn't reached drop-in replacement on the desktop yet. The steam deck is fantastic and I have no problems running it stock, but desktop Linux? Nah. Why would I dual boot when I can stick to one OS that does what I want and need.
(Side note: I run Linux on my server and am quite happy with it).
Windows changed shit up on me too many times, I kept having to relearn where the dick they hid the control panel, for instance. At a certain point I realized if I'm having to put in the time to relearn shit on windows every new version, why not just learn linux? At least I'm learning because I'm getting into something new and exciting by choice instead of being pissed I have to relearn it because some dickhead moved it around on me!
That was actually my experience. My Win 7 laptop died, I needed a new computer, bought one that came with Windows 8.1, and everything was different for no reason, down to whether you left click or right click on the system tray icons to get menus or control panel windows to appear. Switching from Windows 7 to Linux Mint felt like less of a jump, and the changes between versions of MInt aren't that drastic; they don't reshuffle the UI on a whim.