They can't tolerate one another long enough to stay on the phone to complete a trade. The last trade between the two teams was in 1996, when we got Ricky Jordan. The next oldest most recent trade with another team was 2003, when the Mariners needed Quinton McCracken and departed with valuable treasure Greg Colbrunn.
The Mariners have only made three trades with the Angels, none of which are interesting, except that there was a guy named Gary Gray once. (The M's only made the one trade with the D'backs.)
Yes, intradivision trades are rare, but Justin Smoak and Jack Hannahan came from AL West teams. The Mariners have made 7 trades with the A's (we gave them Bill Caudill and Dan Meyer a while ago) and 11 with Texas (how else could we get Tug Hulett, Rafael Bournigal or Ismael Valdez?),