Maybe they figure it would be a waste of time?Seven says: Why assimilate a species that would detract from perfection?
What I don't get is why these undesirable properties couldn't be filtered out. I mean it's not as if all species they have assimated wouldn't have had any less desirable characteristics (e.g. being too aggressive, weak build, etc). I would expect, even if there were no gain in distinctive features, there to be no damage to the Collective either. And they'd still have some extra drones.