Because it's "easier" to support Windows from a business perspective and it's easier on users to use Windows as most already do use it and thus need no additional training/decreases support tickets.
I'm a small business environment it's much easier to manage with Linux but you still need an OK Linux admin on staff.
Once you start scaling up on paper Linux certainly works but there are a lot of factors that most people (such as yourself) don't consider.
This is coming from a pure Linux admin working on a mixed Enterprise environment where 99% of the infra is windows
You'll likely run into some problems eventually but yes, for the most part things work very well now. My only significant problems are typically cause by anti-cheat software.