There are a few reasons for it, IMO:
1. Developer costs.
Even though the install bases of gaming systems are getting larger with each generation, the costs of making games is vastly outstripping the increase in game sales. Look at the most popular console games in history - They've increased a decent bit in sales by units and gross revenue since the SNES days. A good chart would be the UK/Chart Track chart that shows that video game revenue has doubled since the PS2/XB era in the United Kingdom. Problem is, costs to develop a game have gone up more than 2x, and there are more games contributing to said revenue base. So supply of games + cost of making a game is outstripping the demand on the market, leading to prices that are high.
Having said that, if you look at a game that has a very cheap budget - something you'd see from the mid 90's (such as a Braid, Castle Crashers, Final Fantasy: My Life as a King) or Mega Man 9.........Guess what? They're very cheap. Mega Man 9 in 1993 would of cost $50 at retail stores. It's $10 on XBLA/PSN/WW which shows you that the actual cost of games has decreased massively if your going to compare apples to apples (and your really not when comparing content of a top-tier 1992 title and 2008 title).
2. Royalty Fees/Cost of Goods Sold
As the market has increased in size and scope, so has the distribution model of getting games out. More dollars are spent on marketing than ever before - What used to get sales as a "cult classic" by good word of mouth doesn't cut it anymore. Dead Rising, from my memory had a $10 million dollar or more budget on marketing alone. Guess what? That factors in to what the game "costs" to make, because they are spending what they need to make a profit. Such things get asorbed into the overall expense of bringing a title to the market. Likewise, in the distribution model you have more publisher/distribution fees because there are more places that sell products.
3. Used Games.
Did you notice back in the 90's that used games were a rarity? As the market grows, so does the proliferation of used copies. And unfortunately, what used to be a longtail in terms of a game's life has consistently reduced itself year over year, like the movie industry. Used games, as great as they are as a value, have increased their proliferation thanks to Game Stop, to a point that they can choke off poor-reviewed title sales, causing a developer to eat larger losses than expected.
So how will prices go down? Digital distribution. They eliminate problem #2 by giving developers a higher profit margin, and cutting out more of the fees taken by 20 different hands. Likewise, DD titles have infinite shelf life, so marketing costs are lower since WOM is far more critical. Also, used copy issues are negated since they currently don't exist.
And having said that, the Digital Distribution model has proved that it can provide much better games at much cheaper prices. You only need to look at Mega Man 9, Siren, Castle Crashers, Braid, Sins of a Solar Empire, Kings Bounty: The Legend and other great DD titles to know and understand that DD provides a much cheaper deal for the consumer: I've bought Sins and King's Bounty for $40 each brand new. A user can go out and buy Braid or Castle Crashers for $15, and Siren (a PS3 DD title) is $40 as well....Not bad, eh?
Back from the dead, I'm afraid.
