That you think waiting is a job that doesn't require mental presence tells me you have never done it.

Sure, set up and clear down have plenty of routine zone out time - but service is maintaining multiple concurrent to do lists in your head whilst keeping an eye out on the environment for additional tasks.

I was terrible at the job because I have poor short term memory (my lists dropped items all the damned time). But I sure as hell was too busy to contemplate some higher order problem