They did. It isn't explicitly stated, to the best of my knowledge, but the Minds are smart enough that they could easily have done that—and were shown to engineer other parts of the society, such as their deathism.
Humanity has very limited purpose in the Culture. On the surface, it looks nice, but they're effectively pets. Their willingness to ascend was cut off right along with their willingness to live.
All of which is done by social engineering. If you want to go the intelligence-enhancement route, they'll help. They just make sure that very few people do.
