I'd think that's exactly why they want the Varja the way they are (whoever "they" are...)

The Zentradi betrayed their masters in the last television series. With the Varja being remote, mindless killing machines, you don't get that problem. They don't care. There isn't anything to assimilate into society, no culture for them to be attracted to.

They're more machines than living beings, as someone said...just like the Zerg.