And here we have it: the real reason that regulation has been called for, the ability to put out models that don't vastly outperform those previous, while not upsetting (future) shareholders.
I think the real reason is basic collusion. They're burning tens of billions training new models. It's a very competitive space. They know that the companies capable of frontier models is limited so if they can all get together and agree to slow down it gives them more time to make money from inference.
21st century version of the incandescent bulb collusion of the Phoebus Cartel.