My guess is that it isn't fast because there's no reason for it to be fast. There is no reason for that to be a design requirement.
Hell I cannot remember a time when I was _charged for something I didn’t purchase_.
I have been charged MORE than advertised but paying attention while ringing up the item and escalating to the cashier or manager typically solves the problem. I have also been charged less and notified the cashier of the issue.
Why would you check your receipt _after_ you left the store? Now you have to drive back, go inside, go to customer service and wait.
Seems way easier to pay attention and scan the receipt when it is handed to you.
Just to pick one user story, it's a lot harder to check / challenge an itemized receipt if you receive it hours later, once you're already packed things away or eaten some of them.
It's easier to challenge an Amazon Go receipt than any other store. I just click the link in the email and get my money. At other stores, I have to go inside and wait in line behind other people at the customer service desk. I prefer to get on with my day rather than argue over the price of a cabbage.