That is exactly the case. If you pay 10$ an hour for 3 shifts 8 hours each, this makes 3x8x10 = $240 a day. That is $7200 in 30 days per one checkout spot. To setup equipment for self-checkout and maintain it order of magnitude cheaper compared to self checkout. Another factor is eliminating managerial overhead while dealing with human beings, as one store manager told me referring to self-checkouts: "They are never sick, they do not have an attitude, they do not quit and they can't be rude to customers".
Self-checkouts trade one kind of unhappiness for another, and many (most?) are horrifying experiences. There are a few that are reasonably quick and user-friendly, but certainly not all. Most seem to be scraping the bottom of the barrel in an effort to squeeze a few more $ off the purchase price.
If you scanned less than 20/minute you had about a month to get to 30 before you were either canned or lost hours.
Self-checkout is all about cutting labor hours, period. It is both slower and leads to much higher shrink (as much as 150% more) than a manned casher. Stores hope to make more money by keeping the store open longer. The hacks around the slowness of checkout without cashiers is for people optimizing for time is online ordering or shopping at midnight.