It was a default for so long that people just got used to the feel of it and its “ecosystem” if you can call it that.
I use Win at home and at work as my main desktop, because of familiarity, the apps I got used to and because I just don’t feel comfortable with any Linux UI. I get annoyed when the Win UI gets even slightly changed between OS versions, so imagine how it would be for me just switching to Linux. I have a dual boot, but the Linux partitions always gather dust no matter the distro.
But I wouldn’t touch a Windows server. I’m apt with the Linux on work servers, my home server, RaspberryPi and routers. It feeels like having swiss army knives and I feel at home in a command line.
This doesn’t make me a fanboy, but I do get raised eyebrows from co-workers.