I've wondered- or, rather, been bemused and frustrated by this- myself. Not so much wondered; one rarely has to put much speculation into why people don't want to spend more money.
Still, a few things to consider. On a dollars-to-entertainment-hours ratio, a typical novel is still a better deal than a typical video game. But time spent on an entertainment isn't simply a benefit; it's also an investment. Even a fairly long movie doesn't take as long to play as even a relatively short video game, but one feels less cheated for spending two hours and $10 with a mediocre-to-bad movie than $50 spending ten hours on a mediocre-to-bad video game because the longer time investment didn't "pay off" as well as hoped. So some people offset the risk of their "time investment" by not wanting to make as much of a financial one.
Then there's the "fixed in time" effect; the price of games really hasn't changed as much as it probably should have, even as we've gone from one-man games on the Atari 2600 to hundred-person teams on the latest HD efforts. As the gaming audience gets older, so does the market have to deal with expectations that are based on the sometimes decades-long game experiences of the audience.
And then there's the joy of "because we can! Ha!" Game rentals, used games, Moore's Law, stock shift- if you want to game on the cheap right now, it's very easy to do so, especially if you're willing to show some patience and/or compromise slightly on your expectations of a game (that is, not demand a machine-stretching top-of-the-line experience from every new game you put in your machine.) Don't want to pay full price to experience the love/hate relationship your peers have with Brink? Wait for the Steam holiday sale. There's a mass of iPhone games, free Flash games, offerings on GOG, and even free older games that can easily keep you occupied until December. Heck, you can get Daggerfall for free now, and thoroughly exploring that puppy could take you until even Brink made it onto Good Old Games' rosters.
This is not to say all of this is good, of course; I've said before that I think we're headed for another crash of the industry, and some of "our" foibles are very much to blame. The fact is that right now not only can we be petulant spoiled brats towards our favorite industry, but we'll sometimes be rewarded for doing so, as anyone who's only bought a product after it received its final patch can attest. We're well-buffered from any negative effects of our impossible expectations of the industry.
If nothing else, AAA-gaming may have to die a sad death before we realize just how good we had it.