My guess is: progress is stagnating, so they look best if they pretend to slow down intentionally while continuing to work at full speed.
So the “slowing down” narrative is a convenient story to spin.
The entire AI market is propped up on the hope that the models will magically solve all of these problems, and now that even Altman is realising they won't he needs the slowdown to stave of the crash for as long as he can.