Intelligence is also access to data, along with all of it's other requirements. Society as a whole is far smarter than Einstein.

If I dump Einstein on an island at 5 you don't get a theory of relativity. The data provided by a civilization is required, hence why people 10,000 years ago didn't go to the stars. It is unlikely they were significantly less intelligent than us. They just didn't have the language of data we do now.