A few reasons.
Models and decals work a lot differently from each other.
And, the way body parts work is that there's one player model with every body part possible on it, and the game hides all of the ones like hats and stuff unless you select a certain body part.
This is the biggest problem with it.
Of course, we wanted ways to customize hats better, and Rusty and Munk made the Hatmod +.
So useful.
Anyways, if an addon was spread around enough so that almost everyone with a server had it, that might work.
I know Siba was trying to make a copy of the standard player with more animations included in it, with a custom playertype for it, so if you have any ideas, he might be able to help. *shrug*
Of course, it wouldn't work with avatar prefs.
A shoeless blockhead might be nice too. I should go look at that.