As their title suggests, Mew and Celebi are legendary Pokemon, meaning they should, in theory, be extremely hard to obtain. Although cheating devices pretty much ruined that concept, the idea that legendary Pokemon are rare is still Gamefreak's motive of keeping them scarce.
Think about it for a minute. If Mew and Celebi were widely available, what would make them special? Their base stats? Not true, as a good number of easier to obtain Pokemon have their BST, such as Garchomp, Salamence, Tyranitar, and Dragonite. Movepool? Mew may be an exception due its special ability to learn every TM, but Celebi's movepool isn't exactly amazing, although quite adequate for its needs.
Legendary Pokemon are supposed to be hard to obtain, or else there would be nothing special about them at all.
This may be the factor that would be holding people from trading rare Pokemon without receiving a rare one in return.
