It's bad marketing.
If they make games last really long amounts of time, then are they going to sell enough games to make it up stream? Well, very doubtful. People would be too absorbed in the one game that takes so long to beat that they won't go off and buy another game until they need to. Unless a person gets bored with the game.
It's pointless for Sony to make a 200GB Blu-Ray disc if nothing is going to be using the full potential of the storage.
Also, using a longer game thus equates to longer development time, thus equating to a time where there is no revenue. And then, when the game DOES hit the market, the price is skyrocketted for Sony to pay off the people who made the game.
This is why most games nowadays are short. You beat one, and it leaves you wanting more, so you go and buy another game. This goes for all companies, really.
Oh, and, this "miracle PS3 price drop" people are all hyped about probably won't happen even if the price of materials for Sony to make the PS3 go down in the future. Sony's losing too much money ALREADY to drop the price any lower. Sure, you're getting a great deal for what's inside, but is it worth it? Where are the exclusives? Most are turning into XBox titles, as well. Sony's got a big hole to dig out of on this one.