It's absolutely nothing to do with PvP. I don't play on a PvP server and it's incredibly rare that my encounters with other players in the world are anything but cooperative. Even if it's the opposite faction (so we don't share tags) I usually throw a couple of Conflagarates at their mob as I run past.
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@SirCowdog
As has been explained multiple times, when Blizz develop content they have an idea about how long they want the average player to spend each day, and how that time will translate into progression over the days, weeks and months of the expansion.
If they design and pace content with the intent of it being tackled from the ground they can not include flight as most players will likely adopt it as the fastest/easiest/most efficient way to complete the content. This will mean that most players will be experiencing the game in an unintended way, making the time spent developing for the ground largely pointless, and also they'll run through the content too quickly and complain about having nothing to do.
If they pace the content around players flying then players who choose a ground mount will progress at a slower rate than Blizzard intend, falling behind with additional content releases, and also have their enjoyment by "opportunity cost" - in other words knowing that they are intentionally gimping themselves and reducing the rate at which they receive rewards makes the whole game less enjoyable.
So Blizzard had to make a choice, and as they feel (along with the majority of fantasy RPG developers) that playing from the ground delivers a more immersive and satisfying experience that is what they have chosen to develop.