He's a UFA and the Wizards have already made long term commitments to Webster and Porter. Even if Ariza is a starter, those two always present a threat. Ariza is in his prime and this is probably his last chance for a big deal. I don't see him locking himself into any short term, discounted deal with the Wizards. He can hit the FA market for more money, less competition, a better team, and likely some combination of the three. It's in the best interest of the Wizards to trade him while his value is at an all-time high.
The real question is what could we get in return for Ariza? What positions? Can we get an talent upgrade (likely by taking on a contract)? Or do we get a couple of pieces to fortify the rotation?