Because mounts cost too much.
It's really that simple. A brand new character that starts at level 1 and quests to 40, doing random pickup dungeons, not buying any items from the AH, and selling their random good greens/blues will have about 50-60 gold when they ding 40.
Q: But if everyone does that how does anyone have mounts?
A: Sell your tradeskill items on the AH, then you can get mount money
Q: But if everyone does that how does anyone with a leveled tradeskill have a mount?
A: They're making a profit turning materials into finished items
Q: But if the people who aren't supposed to buy anything on the AH to save for a mount are buying crafted items, how do they have mounts?
A: Gold sellers
You can mix and match the above hypotheticals but eventually one (or more) of these groups of people (joe random player, profit-making crafter player, gold saving frugal player) lands on "the only way to get a mount is to buy gold from a gold seller."
It's the same reason that Blizzard had a problem with gold farmers and gold sellers. In a game where every item is bound to the character wearing it, there is no reason for gold at all, honestly. If I had to guess I would guess that Blizzard realized that early in their beta and simply picked something arbitrarily that every player had to buy with gold and grossly inflated that one price. Blizzard had a problem with account theft too, if you remember. People would steal accounts to disenchant all of the items and steal the gold, just like gold sellers are doing here.
The whole thing can be traced back to level 40 and level 60 vendor purchased mounts costing too much.
tl;dr: there's no reason to repeat Blizzard's mistakes just because there's a Blizzard stamp on them. If you want to fix the gold economy and stop gold sellers (and account thieves supplying gold sellers), lower the default purchased mount prices by half and people won't need to buy gold anymore. Instead of having an economy crippled by gold hoarding for mounts, you'll have a working economy in which all sorts of things trade hands.