Most self-checkouts I've seen or used aren't even for people buying more than one bag of groceries, since often the software can't handle it or the bagging space is ridiculously small so it ends up a lot slower for you to bag your stuff in the self-checkout. Sometimes they're not even the best option for when you have just one bag of groceries.

Self-checkouts are faster that going through a normal till when you have a handfull of purchases and there's a line at the normal till because the time saved from not waiting in line exceeds the time loss from an amateur (you) doing the scanning and it not being possible to bag in parallel with scanning (which is how normal tills operate in all the countries I lived in - you do the bagging whilst the person at the till does the scanning so both activities happen in parallel) and even then that's only if you're not buying alcoholic drinks and the software doesn't do something stupid and you have to wait from somebody to come over to clear it.

Only exception are the scan-as-you-pick systems and the ones where everything has an RFID tag and the checkout scans an entire bag at once, but in my experience almost not retailers have such systems and instead have the cheaper crapiest self-checkouts imaginable, probably because they don't give a shit about your time and convenience and only have self-checkouts to save money on manpower not to make the lives of their customers better (certainly that would explain why they usually after adding self-checkouts reduce the manpower on the other tills, making things slower for people buying more than a handful of groceries no matter which checkout mechanism they chose).