check out is the part where the actual sales transaction occurs. It really is materially different

Like a vending machine? Or the gas station? Or the grocery pickup, where I pay online?
What makes a human being present for me giving my money to a machine different if it's a grocery store as opposed to one of those?

Sorry your experience sucks. Stores near me regularly have both open and the self checkout is invariably significantly faster. It's not like I just didn't notice that something I do several times a week actually sucks.