At a certain point intelligence is a loop that improves itself.

"Hmm, oral traditions are a pain in the ass lets write stuff down"

"Hmm, if I specialize in doing particular things and not having to worry about hunting my own food I get much better at it"

"Hmm, if I modify my own genes to increase intelligence..."

Also note that intelligence applies resource constraints against itself. Humans are a huge risk to other humans, hence the lack of intelligence over a smarter human can constrain ones resources.

Lastly, AI is in competition with itself. The best 'most intelligent' AI will get the most resources.