It's not the norm in software development, though -- not that people don't work heavy hours, but rather it seems since every game has a crunch period, then the majority of these games are poorly planned and probably mismanaged.
This is similar to the "lazy developers" mantra, but in the "incompetent managers" form.
And just as I take seriously "Cell/PS3 is easy" only from proven PS3 developers, I accept "game companies crunch, because games are mismanaged" only from proven studios who don't crunch and who make money from their games. (I know studios that don't crunch, that exist purely as a side project of a bigger entity - be it someone's hobby, someone's bad investment, someone's money laundering scheme.)
The crunch I've seen around me happens because of the combination of two factors:
- first, the business model is broken; publishers simply don't pay enough for a game to be comfortably produced without a strain; sales for most categories except the very top of the ladder go down, art budgets go up, the expectations keep going up (try making a FPS without online MP, saved films, coop, Horde mode etc., for example)
- second, most people in a game studio would rather go one additional day to work 12 hours, than see the studio go down in flames. See: young, single males without a life, in joker454's post above. Nobody signs up for years of crunch; it always is just this milestone, which is just a few weeks away.
The one studio I've heard of that doesn't crunch, and kicks everybody out of the door every day at 17:30, is Neversoft; the legend went that they shipped every year their Tony Hawk on the 30th of September to the factory, took one month of vacation, did one month of preproduction, and on the 30th of November pitched the new Tony Hawk to Activision. This is probably why Activision chose them to take care of the golden goose aka Guitar Hero.