Until surprisingly recently at least for Canada those locations were actually even more hostile with ice sheets up to thousands of feet high smothering them. Part of the reason why that area has a crazy low diversity of plants and animals (aside from migratory birds going there to take advantage of low predators in the breeding season) is that basically anything remaining there was killed and it was recolonized with whatever plants and critters from the margins happened to be suitable enough to spread in afterwards. With the ice age other areas were once colder and wetter as well.

With respect to at least Egypt bear in mind that the areas frequently photographed are the dramatic desert parts looming over the sides of the river but they have very extensive lush lands along the Nile which is where nearly everyone actually lives.