There's a good argument to be made from the point of view of customer convenience and expediency for using self-checkouts to pay for a small number of items, but even then most existing implementations of that concept are so fucking bad that there are all sorts of stupid problems, like my case of the thing not working unless I had a bag (it literally had no button to just skip it) and yours were a normal human mistake is complex to correct even though the users are amateurs and hence naturally more likely to make mistakes hence the thing should have been designed differently.

I've actually worked with UX/UI designer at several points in my career, and one thing that pisses me off about most self-checkouts is just how bad their UX/UI design is.

That so many self-checkout implementations are like that is probably explained by, having moved the costs of wasting time to the side of client, those businesses are not financially incentivized to make the self-checkouts efficient to use, which probably also explains all manner of weird choices in everything from their shape to even the order of their menus - in a manned checkout it's their problem because wasted is money being paid to a teller for nothing, so if it's bad they fix it, whilst in self-checkouts it's not their problem so they don't care.

This is also another reason for me to be against self-checkouts: the financial dynamics are different with self-checkouts than with manned checkouts since the costs of inefficiency on the former are on the customer, whilst with the latter the costs are on the store (which has to pay a salary for somebody who is less productive than they could be), so stores have less (and more indirect, hence harder to measure, hence often ignored by MBAs) financial pressure to make self-checkouts efficient to use than they do with manned checkouts.