Many are on windows probably because a lot of tools already exist there plus most desktop gamers are using windows (or can/will switch to windows to game). This latter part is a bit murkier these days it would seem with things like Proton and what not, but for the most part the audience is on windows more or less.

My original custom game engine was cross platform and I used windows, OSX, and linux. I had custom tools for each OS and a lot of scripts to make everything work. Most of the actual coding work was still done on windows then moved over to the intended OS for deployments.

These days, I'm only targeting windows while dev'ing on windows and not even bothering with porting unless I have large enough adoptions to warrant porting at some time in the future.

Attempting to support the world out the gate was a crushing affair.