[bobthepanda]

Most of North Africa and MENA's population centers are located near the coast or major navigable rivers (namely, the Nile). River and sea transport is much cheaper than land transport, even today. Why maintain a bunch of expensive roads when you can just have well developed waterborne trade?

[evgen]

The location for most major cities around the world is where there was a transition point between modes of transport. Sea <-> river, river <-> road, then later adding canals and rail into the mix. Where such transit points exist you are going to develop infrastructure for storing and re-packaging goods from one mode of transportation to another, which will lead to trade of the goods at these locations, which leads to more goods coming in to be traded, and round and round it goes.