If I rob a bank then you can tell. If I make an AI model on my own computer and train it, how would you ever know? It won’t have a “signature” that can be detected by any kind of AI detection system because I’m the only person who knows it exists.

You can make it illegal if you like, but you’ll never be able to enforce it. And other countries are not obligated to follow suit, and will have significant competitive advantages as a result.