Sean Gallardy finds the real killer:
If you haven’t already read up on cooperative and preemptive scheduling or aren’t sure what those are, please read the intro to that first, otherwise you’ll be lost.
Much as I’ve discussed before, SQL Server uses a cooperative scheduling model. This doesn’t mean that Windows does, nor does it mean Windows will scheduler whatever SQL Server schedules, in fact much of the time there are many other threads that run before the ones from SQL Server, that’s the job of the operating system to figure out. Due to SQL Server using cooperative scheduling there needs to be a mechanism that exists such that when a resource not under SQL Server’s control needs interaction the scheduler can keep on scheduling and threads can switch in and out (in SQL Server, Windows does what Windows wants). Enter preemptive status and associated waits.
Click through for a deep dive on the topic.