The scenery for the entire world currently requires 3 DVD-ROMs, which is a significant download for users with broadband access and an prohibitive barrier for dial up access users.
It was hoped that someone would get around to writing a utility for on-demand streaming of scenery to the user, but this hasn't happened. A significant factor is that this streaming real time bandwidth is much more expensive to host than the existing bulk retrieval. Money aside, it isn't a difficult problem.
Suppose we consider the pilot's viewpoint. Most general aviation aircraft cruise below 200 knots and flight visibility is (in real life) usually below 20 miles at their cruise altitudes. The database uses about one megabyte for 600 square miles so the peak streaming rate would be 12 megabytes/hour, less for areas previously visited. A 56K modem is easily capable of 12 megabytes/hour.
The utility for streaming scenery download does not need to be integrated into the core FlightGear source code. The latitude and longitude of the aircraft are already exported for use by independent programs, so the center of interest is trivially available. Since the scenery is stored in pieces, an independent program need only generate a list of the closest elements that have not been fetched yet, and issue a wget to ensure that they will be available before the aircraft gets close enough for the pilot to see them.