I don't think it is just money. I think that many of startups sell themselves to employees via "we are special and superior" feeling. That is way it works best when their employees don't socialize outside of work due to long hours. It is the same in games industry: people like the sound of "I am working on a game", so they put up with worst conditions and pretty much any position - interesting or not.

I've seen this behavior at work environments. It's not explicitly enforced, but if nobody else is leaving the office for lunch, then you don't want to be seen as one who does. Also works with staying late at the office, weekends, etc.