Well that might be because you are only looking at web development jobs. There's more to software development than making websites. Anyone making drivers/firmware will probably have to work on a Windows machine since a lot of hardware need to be compatible with Windows. The whole gaming industry is purely Windows. Most engineering software are written to run on Windows. Any "internal tool" in non-tech enteprise will probably be written for Windows.

C/C++ are still some of the most used language in the world, and a significant share of all c++/c software have been written on Windows machine.