As far as I understand it's because this is a time when merchants didn't exist (except for the foreign traders, who have a restricted set of items they buy in the first place), so trade was bartering away things you didn't have a use for in exchange for ones you could use.
If you had a spare bike and was willing to trade it away, you wouldn't take a spare bike of lower quality and a case of beer in exchange for it, as you'd still have to get rid of an unwanted bike. The NPC is willing to trade away the item, e.g. a knife, because he has another one of at least the same quality.
Of course, you could continue the logic so you'd have to find a person who wants your X, another one who'd take your Y, etc. but I believe that was considered to drive it too far.
This has been suggested before.