I always see self-checkout as outsourcing the cashier’s job to a slow untrained amateur (the client) whilst doing away with paying for that work.
You need to be a sucker to choose to do work for the profiting of others without getting anything at all in return for it (at best, what you get is less slow checkout that the manned tills which they purposefully made worse than before in order to push you into self-checkout, which is not in fact better than what you had before, so not really a “benefit”).
Even in the most purely amoral “greed is good” judgement, it doesn’t make sense to do the work without at least getting a discount.