Because you can buy the other areas with crowns (at least I think you can) so that would defeat their purpose of membership and buying items like areas and card packs etc. it would totally ruin their way of making real money. ( I would also be rich as heck as I’m always at max gold due to all my unwanted items. Lol.)
You can turn most unwanted items into gold by selling at the Bazaar or if it is a "No Auction" item at a regular vendor.
What gets me are the dang 1-Day use mounts. I hate limited use items.... I can't stand using them because I'm weird and I can't sell it because it's not an option so it just sticks in my backpack FOR YEARS!