One reason is so that they can detect generated images and exclude them from future training sets.

There have been some talk of various countries requiring AI images be watermarked. They may be trying to signal their willingness to comply, and hopefully ensure they are not outright banned. That fact it can be bypassed isn't necessarily their problem to fix.

marked images can be excluded from training data in the future, so that it doesn't eat its own output, which is known to cause model quality to degenerate (an issue likely to plague training LLMs on post-2022 datasets.)

as for training the model to add a watermark, that's an interesting idea but I'm not sure it's feasible/not trivial to circumvent.

Many good reasons have been presented here. And on the other side, is there any reason not to do it?

(part from the effort required to put it in place)

Stability, Midjourney, Dall-e, etc don’t want their products to be used for misinformation. They really do want AI generated images to be trivially identifiable.

There are papers around describing how to bake a watermark into a model. Don’t know if anyone is doing that yet.