1) Because they don't practice what they preach. I enjoy questing. I do it even when it's a 'detriment' to my character, i.e. unrewarding in terms of gold/gear/EXP and generally just a waste of time better spent elsewhere. If I truly enjoyed ground travel and did not like flying I wouldn't fly.
2) Because transit in WoW is terrible. My choices are to either spend half an hour on foot trying to get somewhere (make that an hour if it's Stormheim :P) or suffer through a ~2 minute flight path where my character is completely pacified. Both options suck, but one is less painful than the other.
I know where you're trying to go with this too, the 'convenience' angle. However, for me, ground travel isn't immersive either. The most immersive MMOs I've played have teleportation systems and, more importantly, a dynamic world with events. I actually choose not to teleport most of the time outside of quick town runs because I don't want to miss anything. WoW has the exact opposite of that, a perpetually stagnant world that still forces you to travel through it extensively.
I cannot unsee that now, thanks. ;P
Especially as a druid. I'm at the top of my class, yet forgot how to shift into a bird and at least glide?