You pull up. Get out. Put the nozzle in. Then you go inside. There, you wait in line for 5 minutes, because the dick from another pump decided to buy a fucking coffee and a sandwich, and the only employee is busy making those for him, instead of operating the pumps. Then you actually pay and get the gas flowing. By the time you're back at the car, it's already finished pumping.
So, there can be a time gap of several minutes with multiple actions and distractions during it. Is it really that surprising people forget to pull the thing out, occasionally?
Yeah, I know. Most comments in this thread say the exact same thing.
However, it's obviously not the case everywhere.
The OP didn't exactly specify where he lives and only said that you have to pay "at some point", so I'm giving my point of view.
Obviously it's harder for forget it you have to hold the thing, but that thing isn't universal.