Oddly enough, one of the things that I most enjoy about Egypt is probably something that would drive other people nuts. People here do all sorts of things that would never be permitted in a more organised, regimented country where someone is looking out for everyone else's safety. You see people riding in the back of trucks (forget it in North America...would void your insurance) all the time. Is it dangerous? Sure. If someone is injured or killed, obviously people are upset, but there is a basic realisation that we are all going to die at some point, and that we have choices about how, when and where. Make a stupid choice and you can get a stupid death.
These men are probably more comfortable in the short run sitting on top of the load of their truck than they would be in the cab...and there probably isn't room in the cab anyway.