Ancient Egypt was also rather low on end of the world proof bunkers...
There are only so many resources to pick from, visually, if you want to be 100% consistent to the architecture, the location and a historically accurate representation of society 5000 years ago. Z is very good and improving with every release, but he is not going to create renders from scratch, designing his own 3D environments and objects just yet.
So we do operate within the constraints of that limitation, and that is the practical side of it
That said, it IS consistent with the in game lore.
The game takes place in a dystopian future.. a future where a bunch of people with a solid knowledge of ancient Egypt and a bunch of Egyptian ruins laying about did the best they could to rebuild a society modelling it on a commonly shared knowledge of what seemed like simpler, more manageable and straightforward times.. but they themselves were not original ancient Egyptians, in fact they were a multicultural bunch, so they blended in other things because, some things developed later just work better and are easier to do/fix/work with with limited resources.