One possible reason is that eliminates a whole category of unskilled labor? (full disclosure, writing this from Oregon, where we can't pump our own gas to preserve another category of unskilled labor jobs - except right now, thanks COVID-19?) I personally would probably prefer to have a human checker for that reason, I don't notice much improvement in speed or ease of checking out when I do it myself, there's just a line for the self-checkouts instead...