Out of those I think Gaming might be the biggest, and some games like League of Legends (all Riot games) need Vanguard, their anti-cheat which only works on Windows. So it's not easy for Valve but hopefully it will get there sometime.

The biggest reason I don't just migrate is because gaming. Most steam games could work on Linux but then if you want to play one that doesn't you have a problem. I'd rather just use Windows and never have a problem, because the game was designed for my platform.

That being said, for work I use macOS.