Well, when a player scams another, it's creating a minor priority for economy, what is it you ask?
At first, we wouldn't need anything droppable due to players stop buying sold items. Another thought has came in mind is doing 'certain' items droppable just for the cause of 'gifting' certain items to friends.
Scamming is supposed to be illegal, but due to 1000+ players and 2000-3000 drops a day, the log (if we had any) would not be able to locate everything done, and staff does not have enough time to check the real/fake complaints about each player scammed, so we considered it legal.
Why don't we want an trading system? Since scamming is legal, it will detain the player from trading (making him scared somehow), which will obviously remove the fact that it could be traded/dropped to anyone ~ which, in the end, will make the other player obligated to buy/earn the item.
Having a trading system makes the player relaxed that he can actually trade or buy the item from/to anyone. Which makes him relieved from earning it.

This concludes this whole reason why a trading system and having all items droppables is a bad idea.