Because we're talking about add-ons specifically. Yes, you can buy a downloadable game for about the same price. For add-ons, however, you need to have purchased the original game in order to be able to play DLC like Dead Money. That's what make this DLC that we're specifically talking about "special". Make no mistake, I'm not defending the pricing of GRA. I'm simply pointing out that the price for this type of DLC can't and shouldn't be compared directly to full games.