From my personal experience, scanning things by yourself instead of more experienced cashier is somewhat slower (maybe 20-40% for large amounts?) for reasons you provided. The thing is, you don’t have to replace one cashier with one self-checkout, instead you may put like 5 of them and assign one employee to supervise them and solve things that need intervention like verifying age. Also when not in use (low amount of customers) they probably cost tiny fraction of employee’s wage. Idk about thefts though.