The machines also break, use electricity (possibly more than the clerks with their registers and belts and lights), require infrastructure to support (e.g. price changes must now also be updated in their possibly-separate pricing database, also consider stores labeling prices by hand or only on the shelf), and can't immediately replace 100% of cashiers (some customers will prefer human interaction, some states e.g. require that liquor is sold not by a machine, etc).
It does also create needs for different jobs. Instead of 10 cashiers @$10/hr, you now need 1 supervisor @$16/hr + a consulting company to install and integrate the machines @$300/hr + technicians to re-stock the machines and service them @$90/hr + customer support to help with software bugs in the new pricing system etc etc. Of course, you can then go and automate all of those jobs as well, but it seems a bit fractal until we have AI strong enough to build more AI where it's needed and consolidate existing AI, which I guess is what people are supposing leads to the extinction of humans since we're so inefficient.
It does also create needs for different jobs. Instead of 10 cashiers @$10/hr, you now need 1 supervisor @$16/hr + a consulting company to install and integrate the machines @$300/hr + technicians to re-stock the machines and service them @$90/hr + customer support to help with software bugs in the new pricing system etc etc. Of course, you can then go and automate all of those jobs as well, but it seems a bit fractal until we have AI strong enough to build more AI where it's needed and consolidate existing AI, which I guess is what people are supposing leads to the extinction of humans since we're so inefficient.