Is the theft really happening at the checkout?

And if so, why can't we detect it via camera + AI?

Detecting theft does not mean theft is prevented. You then need the government to prosecute, and impose sufficient punishment to deter theft. This is not cheap, nor a given that it will happen.

You detect someone leaving your store with a 4€ item. What then?

You ban them from coming back in after a few warnings. Stores seem really icy about facial recognition right now though. The optics are pretty bad (a play on words pun?)

Who is going to stop them from coming back in?

There have been a few stores that won't really stop people from coming back, they just quietly file charges, and then the person finds out next time they get pulled over or something along those lines.

No one, they get automatically flagged, and then someone asks them to leave. Or the police are called and they are trespassed.

You install AI-powered turnstiles at the entrance. Come on haven't you seen or read any dystopian media? :)

Use your army of lawyers to help file misdemeanor theft charges of course. Then get one out of 100 of the defendants who actually has something to lose to pay big damages to fund it to happen again the next time.

So take the broken god awful experience of self checkout and add another layer of “I think you did something wrong so now you have to stand around waiting for an actual person”?

No thanks.

There are stores that are abandoning self-checkouts completely and going back to cashiers as the theft rose to unsustainable numbers.

Checkouts are often only egress points. So having pair of eyes over them does have some effect compared to having none at all.