The hard part is preventing theft, not adding numbers.
I'm sure you can find videos of thefts in San Francisco if you need a visual demonstration. No cashier is going to jump in front of someone to stop a theft.
If there's no cashier and you're doing it yourself, a whole lot more people will "forget" to scan a couple items, and that adds up.
You're doing the all or nothing fallacy. The fact that a cashier does not prevent all thefts does not mean a cashier does NOTHING for theft.
Yes, for one thing, it ignores that a very large share of retail theft is insider theft, and that cash handling positions are the largest portion of that.
Cashiers absolutely do something for theft.
And if so, why can't we detect it via camera + AI?
No thanks.