the ui the cashier gets is one that is effectively for an expert; they have to be trained to use it (and develop muscle memory), whereas the self-checkout machines are simultaneously nerfed for perpetual n00bs and fortified against theft, making them inherently less efficient
so in a way you could imagine the cashier-cum-self-checkout-attendant being themselves frustrated to have to help frustrated people limp through those things when they erstwhile could have just done it for them using a professional rig