It is 100% implementation. In other countries there’s either a staff member watching over all the self checkouts to make everything go smoothly, or a kind of electronic gate that only let’s you leave after scanning a receipt. Usually the scanners are much more reliable and theres a usable UI. Plus a modicum of trust. Also thise hand scanners you can carry around the shop so you don’t have to do it at the end (although I think if seen some of them around now).
In the UK there’s usually the weight detection mechanic that slows things down 10x and no interactivity with the machine other than it loudly telling you you’re doing it wrong. You often need to ask for help anyway.
If it was a quick and easy experience the scheme wouldn’t fail.