Their work is impressionistic. When a game director or Activision asks for a player unfriendly system to be developed they have something specific in mind, you're not allowed to take liberties.
Art on the other hand is the exact same no matter who's in charge. Someone asks for something, you interpret what they want, put your spin on it and come up with a suggestion.
Essentially the artists can use their own judgement and creativity to come up with something the players will like, regardless of what the original request was.
A developer can't do that. If you're told to create azerite you have to create azerite, you can't deliver valor points and expect it to go down well.
An artist on the other hand can be asked to create a castle made out of a giant pumpkin, but deliver one made out of a giant eggplant instead and it'll probably be very well received if the work was really good.