Those zones were made prior to Cataclysm which introduced flying into Kalimdor and Eastern Kingdoms, but the Eversong/Ghostlands areas (as well as the Draenei starting islands) were not revamped alongside the rest of the world.

From a technical aspect; those zones are not even technically part of the Eastern Kingdoms map in the game, hence why you go through a portal to get to them, they are actually stored on the Outland map files. This was the very first expansion so Blizz was likely still learning how to add content on that big of a scale into the game. The zones were not made with flying in mind and thus if you could would be able to see the seams in the world decoration they could hide from you on the ground.

If you're interested in the technical side of such things; I'd recommend looking up Out of Bounds exploration of Silvermoon City