Partially this is due to the fact that almost any creative task requires a certain degree of focus, and the more people you attempt to include in the design process the more effort needs to expended keeping the project on track. There's a reason projects of this magnitude often have people who specifically work on doing exactly that. They don't do the individual art, they don't code the abilities or design the environments or that next cool armor, they instead work on keeping all of these variables on track. They're jugglers, and the balls in this case are the varied and disparate elements of the game's overall design.
Attempting to allow the millions of players to, as an example, vote on the game's design in some sort of plebiscite would be a nightmare for a variety of reasons. First, it would make managing the size of the project almost unthinkable. Second, it would dilute the vision behind the game. If you're familiar with the auteur theory of film, imagine attempting to keep any sort of consistent vision for your game with millions of voices clamoring on design issues. Combined with the fact that playing the game doesn't translate to knowing much at all about designing one, you risk significant pitfalls for minimal reward. There are several reasons pure democracy is an extremely rare form of governance today -- it becomes extremely unwieldy once you pass a certain population. World of Warcraft and its player base has long since passed that threshold.
Am I arguing that the players and their opinions don't matter? No. Absolutely not. That would be foolish and absurd, and while you may believe me to be both of those things and I wouldn't argue too hard with you, I'm not indiscriminate about either. The likes and dislikes, the expressed tastes and the satisfaction of the game's players is of crucial importance. But again, just because we know what we like doesn't mean we know the best way to create what we like, especially while preserving other elements of the game we like. The very size of the game's community precludes anything as simple as pretending it speaks with one voice on any issue.
Some of us are primarily focused on PvP, others would rather never set foot inside a BG or arena, still others dabble. If a community vote was held and it turned out the majority would rather see PvP removed from the game, that majority would be happy with the decision, but a significant minority would probably stop playing altogether. And some of that minority might be in your guild, could even be your friends in game. The community as a whole would be weakened by their loss. And this is just one example. Character models, class balance, these are issues that have wildly disparate opinions scattered throughout the community as a whole. It's actually better for everyone concerned to have the developers working as a focused, directed group to deal with these issues instead of throwing them open to a vote. For one thing, in order to ensure the fairness of a vote, steps have to be taken to ensure there is no tampering with the results, something every nation, state, province, city, town or other governmental body that holds elections has to deal with. Do we want Blizzard having to set up a vote fraud policing department? Is that a good use of their resources?
The fact of the matter is, mob rule isn't going to design a good game. Not only are there too many of us, and not only do we not really agree on anything well enough to produce a coherent vision, but the game has succeeded and continued as long as it has by attempting to find and cater to as many disparate playstyles as possible, some of them very much not for everybody. The game benefits from listening to us, absolutely. It benefits from hearing our voices, and seeing what the wide variety of opinions is. It does not benefit from any attempt to try and design the game by us as a group, because as a group we simply lack any sort of definitive focus. We don't want the same things. We don't share the same interests.
To use another analogy, you might read a book written by diverse hands. One of my favorite series of novels is Wild Cards, edited by George R.R. Martin. There are a lot of authors working in this setting, and I enjoy the books for their wide-ranging selection of voices. But would I enjoy the series if it had several million authors? I've seen enough fanfic in my life to doubt it. Sure, there's good fanfic, but the signal to noise ration isn't reassuring.
The proper use of community opinion isn't to design the game, it's to suggest where the game needs improvement. Should we say "We're not satisfied with X"? Absolutely we should. We should go into great detail about what, exactly, bothers us about X. We should offer exactly why we think it breaks down, and making suggestions on how it could be improved certainly could be part of that process. But the danger of passionate fan commitment to anything is that temptation to believe it belongs to us, that we can and should be running it. The fact is, we're the game's audience, and that's worthwhile. The game wouldn't exist without us playing it. It's fine and good that we are its players, and while some of us may one day be working to create it, that doesn't mean the game should try and allow us all back into the kitchen. Several million cooks in one kitchen would not make anything worth eating.