Because the idea that if you don't want flying then you can ignore flying is a naive argument that avoids the entire debate of why people don't want it in the first place.
Asking people to avoid flying is like asking players to write their own story. Or asking players to avoid shortcuts.
At best it is the developers admitting they don't have faith in their own product, at worst it is them actively sabotaging themselves.
Imagine for a moment that there was a gear Vendor in the game that gave you all the gear you wanted for free. All the content was still there and you could still gear up the normal way, but if you wanted to there was an easy way to essentially cheat and skip all that pesky gameplay and instead go straight for the ending.
The core of the issue is that flying in WoW is not a balanced mechanic, there is no way to use it that doesnt have you avoid playing the game. There is no inherent weaknesss, there is no inherent cost, when you use it there are no obstacles the game that reasonably give you except taking flying away.
Compare for instance to flying in GTA. You need to find a flying vehicle, you then need to take off. If you stole the vehicle then NPCs will try to shoot you down. Once you have the vehicle in the air you can in theory go anywhere, but landing is close to impossible, and landing in such a way that you can reuse the plane even more so.
However you can also take a car, which has less weaknesses, but is also slower and cannot avoid every obstacle.
GTA however is a game desigend to just have consequence free fun, and even that game understands the importance of not giving the player the power of infinite flight.
WoW is a game where the open world content is actually supposed to have a baseline level of challenge meant to be an obstacle to the player the entire game, and in this game you get essentially infinite flight. The logic behind it is actually baffling when put into context of other games with flying.