I don't think the blame for the workaholic culture lies primarily with VCs, though some do encourage this. I think it's mostly a part of the cult of male ego. "I worked 70 hours last week! How many did you work? 50? Wuss! I'm so much more hard-core than you!"
People also lie about how many hours they worked. In reality they work maybe 20-40 and then do things like... uhh... hang around in here... for the other 20-40. Again part of the macho one-upmanship thing... putting on an act of working ridiculous hours to look hard.