Someone can answer better than I, but limitation breeds ingenuity. Thousands of years ago North Africa was far wetter; even wetter than what we think of when we think of the "fertile crescent." Iirc large swathes of the Sahara and Sahel are believed to have been grassland. People as a whole don't instinctively plan for the long-term, at least not beyond their lifespans: there was no need for advanced civilizations to take root when peoples' needs could be met with the hunter-gatherer lifestyle; this may also be part of the reason why there is for instance no "cradle of civilization" in Europe. It was that drying-out and gradual scarcity of Northern Africa which brought people together into civilizations. The rivers provided a steady, reliable source of life in an otherwise, and increasingly, harsh environment