As in, they couldn't afford to put water in the ground for food later because they need to drink now. Nor could they always afford to stay in one place as they had to follow herds for food today rather than later. This isn't 100% the case all the time, but simply having fucking water, and with it fertile land another major lacking in the rest of the continent, is a major reason why Egypt is such an outlier in the continent even before Europe got involved.

Plus the water they did have are breeding grounds for malaria, practically dooming any attempts at irrigation. You still have Africans today that refuse to drive on bridges over water out of fear of "evil spirits".