I'm going to take a wild guess and say that it's time. We're paying for time.
What may take only 5 months for a movies may take 2 years for a game(for reasons unknown to me when it comes to dedicated Game developers whose job is to make games...I can see a hobbyist though)so I'm guess we're paying more because it takes longer to make a game I guess.
Funny, I agree that we're buying time, in a sense, however I disagree that we're paying for
development time. Your average movie lasts around 2 hours. Once you've sat through that it's over and done with. That's it. You've seen everything there is. With a game you
usually play a lot longer than two hours (unless the game sucks ass) and
usually include something to add replay value (new game+, 100 percent completion, etc). You're essentially buying, like, three times the "time" of a movie (?). :V