It's trivial (if you know what you are doing) to load kernel drivers from userspace by abusing windows bugs.
If you trust a program to run in userspace, then it already has all the keys to the kingdom.
Welcome to windows security.
Strange, I can do the same in Linux.
Welcome to GNU/Linux security.
A lot of the times you won't know unless you go digging through review comments or people have made a fuss over it.
To some people, the priority is the game and they couldn't care less about the borderline malware required to do so.
Most people installing these games don't even know what a rootkit is.