Could it be that these terrain maps are compiled from hiking maps, which tend to be very local and don't label obvious or general features like "the Nile" or "the Alps"?
I can't think of a single hiking map I've ever seen that didn't label rivers, no matter how "obvious" they were.
To me, "terrain map" means "physical map". If the "terrain" maps are intended for hikers, Google should instead label them as "hiking" maps. But I doubt that they are intended purely as "hiking" maps... none of the trails near my house are on them.
Compiled from hiking maps? In places like Egypt, his first example? My guess is that there aren't a great deal of hiking maps for the Sahara desert...
They're compiled, I think, from whatever they can get their hands on. It's interesting to see borders, for instance, where there is clearly a difference in the data available. Switzerland, for instance, seems to have really good data:
To me, "terrain map" means "physical map". If the "terrain" maps are intended for hikers, Google should instead label them as "hiking" maps. But I doubt that they are intended purely as "hiking" maps... none of the trails near my house are on them.