Well answer me this how many of these people sell these fash and mounts rather than horde em and say I love it too much to let it go. If they truly love it too much to let it go then it won't matter if others have the item too they still won't let it go
Part of their "love" is the fact that it is so rare, hence why they don't want to let it go.
Not always
Then what other reason would it be lol? It's not like they are greedy, they already have it and aren't selling it for ridiculous prices. What makes a collector a collector has to do with how rare certain items, their increased love for a item has to do with rarity or sentimental value. More often than not a collector would be more proud or in "love" with a item that is more rare than another. For instance I love my blue hunter more than my black spider silk, but I wear the black spider silk cause it looks better to me. Hence the rarity increases love for an item.