That’s because working as cleanup crew or on the ramp is basically indentured servitude, and there is a single ATC facility in the entire country that is fully staffed. More controllers are retiring than are being hired.

Things impact each other, if you miss your slot or you time out for the day you’re done, go home. Your airplane will be taking up someone else’s slot and they have to move it to where it’s supposed to be and find the crew to do it, and it just cascades until gridlock and a hard reset.