Chimpanzees are well adapted to their environment. Their environment has changed little so they have changed little.
Some of our common ancestors stayed in the trees, others were driven out by some mechanism, perhaps they were crowded out into the grasslands, perhaps their local forests died out due to drought or some infestation.
The ones somewhat better adapted to looking over the grass to see predators and opportunities had more kids. The ones who happened to be better at holding and using tools had more kids. The smarter ones had more kids.
It doesn’t take all that many generations for a change in selection processes to change a population.
Or maybe it was triggered by the "room-for-thought" mutation, leading to an "evolutionary cascade".
That hypothesis is as good as any other.
Bottom line is these are all hypotheses at this point, and anyone who claims otherwise is blowing smoke.