When most people request beards its for the stubble variety which Yoshi has addressed before. In short there are some technical challenges in making stubble look good on all the different facial types; they'd almost have to do a beard for each facial type and they don't really have the back end space for all of that.

I'd imagine the database space limitations they're working under would apply to big beards as well but it would probably be easier to do a "one size fits all" beard with a big beard.

But I suspect this is way down on their priority list; its been an oft requested feature since HW and the devs know of it yet nothing has happened.