Apr 12th 2007 4:14PM "Why they don't retoon azeroth a bit for flying mounts, I'll never know....oh wait...I do know...lazy buggers."
No - its not laziness. The vast majority of Azeroth is modeled in 2D - with some areas/buildings being more like "1D". Kinda like a film set - its just a facade.
The Outlands however, were completely modeled in 3D because the requirement for flying mounts was designed in.
To allow the 'world and his dog' flying mounts in Azeroth would require humongous effort and time.