It is really ridiculous.
Activisionblizzard has a net worth in the ballpark of twenty billion dollars. They could easily get as many artists as they need from other companies just by waving some money around. Moving to different companies is part of the industry. Hell, there's loads of unemployed video game artists right now (market is very over saturated) that can do excellent work on the cheap.
There is no excuse. They have the means, and the ability to get this done in a very timely fashion. But they don't. Its pure laziness. You can bet your ass if there was another MMO actually providing some heated competition to WoW they would have been all over this years ago. However because there is no other game to provide real competition they have become content to sit on their laurels and vacuum in cash while dipping as little into their profits as possible.