Teef are stable, and there are no teef to gain from conquest; Orks do not go to war for profit motive (nobody else has decent teef, with the possible exception of Tyranids, who have more teef than anything else in the universe. The encroachment of the Hive Fleets may well cause a devaluation of the toof, and severe damage to the Orkonomy, if anyone ever beats them). Orks go to war for war's sake, and do not so much care about the profit involved, although they will of course engage in lootin' once the enemy are all dead, and may well trade looted stuff to the Mekboyz. Orks gain pleasure from fighting, not booze, and there are no Ork women (women being the principal reason for economic activity in a race that features sexual dimorphism). The only real reason an Ork would have for spending teef is gunz. Just as Heinz meanz beanz, Gunz meanz Mekboyz.
Mekboyz are responsible for the weapons, vehicles and other equipment of the Waaagh. Mekz were programmed by the Brainboyz/Old Ones to instinctively understand teknologee (Orkish force fields, produced by untrained savages, are comparable even to the Necrons or Eldar) Mekboyz, too, are not driven by profit motive or by greed for food or booze. Their motivation is to outdo other Mekz and make the blastiest kustom shoota, the fightin'est Dread, or the zoomin'est (red) trukk. Other Orks trade them teef for their gunz and trukks, but what do Meks do with their teef? More scrap and more guinea-grots to continue with their work.