I’m sure its not a case of not being able to fit the actual maps into the memory of a phone, more so the issue of the added cost of doing it compared to keeping all of the maps in a central database that people access when needed.
Course a system like Googles navi has got to require massive amounts of memory with all the different map views and settings it has possible, so its prob a combination of both?
Would be cool if you could download your route that you have planned so its all there on your phone in case you lose signal somewhere along the way. (assuming that this isn’t already possible.)