TL;DR: because you need professionals for modern CG and programming, professionals deserve good pay and you need a lot of them, also considerable marketing is required for non-innovative games.
I've got a feeling marketing is eating up most of the budget for all AAA titles. I can understand seeing how consumers seem to love the hype train. Does anybody have a clue on the actual size of marketing in big productions? I couldn't find any facts.
And there's some random numbers throughout history in this article, such as Modern Warfare 2 costing $40 to $50 milion to develop and a total launch cost of $200 million. http://kotaku.com/how-much-does-it-cost-to-make-a-big-video-...
And yes game dev is a lot of hours for less pay than your fantastic Silicon Valley wages.