Because I am not the owner of my work computer. And still there is productif proprietary software that is more effective to use than opensource alternatives. And often this software is first written for windows.
Discussion
Same with my work computer. Should have specified that it’s when you own it. If “you” is your company, same question applies. Proprietary software requiring Windows is a valid answer, but perhaps much of it works in Windows emulation (Wine) on Linux?
I am only privatly working on Linux. And do not do big Projects to design electronics or CAD-stuff. I think one would have to try how it works. But I totally support use opensource whenever possible and only use propriatary on the highest layer necessary. To give the lowest liberty to proprietary software for privacy purpose.
But always have on your mind that with FOSS updating is even more important then with propriatary, since the fixes are public as well.