Why? It turned out it worked well for us, so it continued to develop even further. It's like asking "why the neck is strongly developed by only giraffes". Additionally, intelligence is no on/off switch. There are different species with wildly varying degrees of intelligence. Crows, dolphins and apes are only the most widely known examples for this.
Today morning have found a spider in my room. I do not think he knows more math then me yet he is able to make mathematicaly perfect web. Also he can make the web which I cant so there my have been some time in the past where making webs didnt turn out so well for humans?
I have learned that humans have actualy nonactive genom in DNA for bat-like wings but for some reason "they didnt see it necessery" to activate it...