Because I care about leaving, so I do everything I can to be faster. In economics, this is known as the principle-agent problem. At my local walmart, it is known as "I'm not a septuagenarian who's been hitting a vape pen for the last 5 hours."

At my local shop, some of the cashiers are extremely slow at bagging… Often I end up when bags that are way too heavy, and sometimes my bread is all smushed. I don’t fault them, I can’t imagine they’re being paid a reasonable wage.

I am absolutely faster doing it myself.

Amusing that you think the employees scanning shit aren't also the ones bagging it.

But to answer your question, I'm faster because I have an incentive to get shit scanned and bagged, vs just riding the till for 8 hours.

Hint, they're probably not. They perceive themselves as faster, but on average the employees are.

Because I worked as a package clerk as a kid, some 30 years ago. They spent a week training us to be cashiers and how to pack groceries as optimally and quickly as possible. And most places around here, the timing of the cashier is not good, especially since we usually have to pack our own groceries anyway.