Neither per se. They have mentioned before one of the main reasons is zone geometry is not open sky, and there are many 'invisible barriers' in the sky, and structures with one sided textures (so you could see mobs from above that 'can't' see you), or no roof at all. Among other things.
Flying mounts in WoW was a mistake. Gliders in archeage were pretty fun without a lot of the negatives flying mounts have