One of the core elements in MMORPGs is the ability, to a greater or lesser extent, to customize the in-game look of one's avatar.  Yet this feature has not been successfully implemented by any of the major console FPSs that focus on multiplayer (at least I hope someone doesn't point out a glaring example that i'm forgetting right now).  

I'm not talking about superficial changes such as repainting the skin, but rather altering the costume entirely (think Battlefield Heroes).  What factors have prevented this addition that would be immensely popular?  Processor Budgets, Time/Economic constraints on the production schedule?

Just wondering...