Maybe it's the customization? If all I have to do is scan things, then I can do that about as fast as the professional checker, but if I have to make customizations and select from menu options and so on, maybe it's faster, and certainly easier, to let the professional do it.

I wonder if McDonald's, or similar, is taking the audio from their drive-thrus, and the resultant order, and using that to train a machine learning model. If you could train an ML system to be as easy to order from as the standard cashier, and combine that with a screen and an intuitive (and standardized) interface to modify the order, perhaps you could bring more people to self-ordering.

“the great resignation” is a cute catchphrase but misleading. There was basically no net voluntary withdrawal from the labor force, just a rapid resolution of deferred workforce mobility resulting from both sides (employer/employee) being reluctant to start new jobs during the peak of the pandemic and associated restrictions.