There's also the reason we aren't supposed to talk about.
It doesn't phone home so you can use the same key on every machine you build.
It might as well be a free OS when used for this purpose.
I'm not wasting a good windows 7 key on an outdated PC to play 30 year old games on.
My cab uses Vista because I had a copy and needed DirectX11 support.

I probably will be updating that to Windows 7 soon, but that machine is connected to the internet, kept updated, and runs Steam games.