Probably the pollution. A lot of AI companies are trying to find ways around it, by including watermarks in their model output that their tools can detect, and so on.

We already know that training models on themselves causes model collapse, so OpenAI deciding that they've had quite enough wouldn't be surprising.

On top of that, a lot of websites are clamping down on access because of scraping concerns. Reddit shut down a lot of their third-party APIs, and nicer features, like RSS, Old Reddit (if you haven't used it in 6 months, you lose access to it, and you need an account for it), etc. Google seems to want a captcha on every search. Twitter famously ramped up the price of its API immensely,.and now has ridiculously stringent measures for accounts, like needing users to download the app, and give hand-scans. It wouldn't be at all surprising for OpenAI to decide its not worth the effort at this point to try and get around all that, nor pay for it, when they've already got all the previous data. Training doesn't destroy the data used.

A lot of AI training data these days is generated synthetically anyway, and that can be done in-house, whether by their own models, or downloading an open weights model and running it themselves.