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).