In my household it's MS Office. My partner very much enjoys the UX of KDE, but can't switch because she's highly reliant on features in MS Office that I cannot find transliterated well enough to work on Linux. Otherwise, there is no place for Windows in my house.

Personally? Inertia. I hate switching tools/systems unless absolutely necessary. If I were a frog you could absolutely boil me.

For me? Gaming and lack of alternatives. I hate MacOS' hostility toward anything not Jobsian, and Linux doesn't have parity with Windows particularly around gaming.

I prefer Linux, but I dual-boot Windows for gaming, since I haven't been able to get my Nvidia card to work at its full capacity on Linux.