Because they are commodities with value that should not just be given away. what else are you going to trade them for? the cubs have 1b, 2b, 3b, lf, rf, cf (pie), four spots in the rotation (prior, hill, lilly, marquis) and a bunch of pen guys (dempster, howry, eyre, etc) signed for next season. some of those spots are taken care of for the next 3-5 years.