Because I understand how trivial it is for blizzard to shift a handful of bits and make my character level 90.
Just because something is worthwhile doesn't mean it's not over-priced. Let me give you an example.
A mage in a city is selling portals for 100 gold per portal. Now let's say you are well-geared, and skilled at the game, and you know in the time it takes you to get from where you are to where you want to be portaled, you could make at least 100 gold, probably more. It's worth your time/money to buy the portal, but that doesn't mean it's not a greedy pig-fisted price to charge for spending 10 whole seconds casting one fucking spell.