It's just marketing, plain and simple. Video games and other software companies are almost forced to take this approach to make money. It's not like Coke... "Buy our product, the same one we've been selling for years," it's "this is the hottest new title and it renders all past titles obsolete." Honestly, would you buy something that says on the box: "This game will have a sequel out in four months that will render this product unnecessary for you to own?"
By that same token, and all things being equal, would you sell a product for $45 when you can sell it for $75?
I'm not sayin' that it's ethical, mind you. It's just one of the many luscious fruits of capitalism.