Because the first one to name a figure gets their options cut off. If I say "How about $80K?", I've just cut off hope of any higher. They know I'll work for that, so there's no point in offering me more. On the other hand, if I ask for a figure they're not willing to pay, I'm running the risk of being considered too greedy and having an inflated view of myself. If I happen to know their high end, I can start there, but otherwise I'm likely to come out worse. Similarly, if the interviewer suggests $70K, then I know I can rely on that and negotiate up.
If you're a techie, you're probably a worse negotiator than your interviewer, so you're likely to be maneuvered into offering the first figure. Try to avoid that. Something like "I make $70K already, and I'd like an increase" will at least preserve some upward negotiation ability (of course, if the company isn't going to be offering that, you don't get the job, but that's probably what you want at the moment).