If you check out the trade blocks you'll see a lot of older players in there which puts a lot of people off wanting to trade for them and that market is stagnant as you may only get one season out of them, and still good players. To try and stimulate this market would it not be a good idea to give teams compensatory picks to try and encourage owners to make these moves?
I know that all of the players in the trade block page that are 10+ years are tradeable but just like you said, there isn't enough to really trade for them.
Cmon people, this is a football management simulation, were supposed to simulate real life! ;)