Because their customers use Windows, and at some point, even when their app is multi-platform, they need to work on the platform their customers are using.
Amateurs can do as they like, but professionals need to follow the money, and sometimes, Windows is where the money is.
Edit:
Windows has a few strong points, I particularly like the Visual Studio debugger and there is no equivalent on Linux (my platform of choice) that I am aware of. But really, since you specially mentioned professionals, the answer is simple: money.
Amateurs can do as they like, but professionals need to follow the money, and sometimes, Windows is where the money is.
Edit:
Windows has a few strong points, I particularly like the Visual Studio debugger and there is no equivalent on Linux (my platform of choice) that I am aware of. But really, since you specially mentioned professionals, the answer is simple: money.