From a game mechanical perspective there are two reasons:
- It encourages crafting instead of scavenging gear and makes sure that I can give the enemies special gear without also giving it to the player.
- If the bound system didn't exist, you'd never need to craft more than 4 of each item as you could swap them between parties. While that would be extremely annoying to do, players will still do it, so it's better to prevent it altogether.