Agreed, there's often not enough incentive for companies to not do it, and people don't quit often enough - both were true in the cases where I've worked lots of overtime.
That said, productivity during extended 80 hour week crunches may well dip below 40 and be a net drag on the company, but it probably still wouldn't matter. Game studios in particular aren't asking themselves whether it's worth it to crunch, they don't know whether crunch is more or less productive and they don't care. They look at a looming deadline that appears to take 3x more work than there is time left. Publishers attach bonuses to being on time, and fees to being late. Crunch is often seen as the only option. That shouldn't be true at the huge game companies, but it still is.
Plus, it is really hard to measure true productivity in a non-factory setting like software. Nobody actually knows if 80 hours per week sustained equals 50 or 30 or 70 or 10.
All that said, a true 80 hour work week doesn't leave enough time & energy to eat properly, see your family, or even watch TV, let alone job hunt. I'm the kind of person that always does extra work, or has side projects going, but I realized at some point during game crunches that I was trading money for my entire life, and nobody can pay me enough to work 80 hours a week.