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.