Their common sense? I think avatars would lose their value or distinctiveness if its easily tradeable and accessible by everyone.What’s stopping ATA from making old avi boxes/bags tradeable, or avi boxes in general? They’re boxes.
Think of the avatars as some sort of an achievement reward. Its common for games to have a unique item that was only available at a certain year or event. IMO its like a way of how the developers give appreciatiation to the players who were already here before the newer ones. I mean, everyone likes to have rare things. But it wouldn't be called "rare" anymore if everyone has it now, would it?