Besides the overall Disney mark up, those prices are also highly inflated so people on the
DDP can feel like they got their money's worth. The DDP increases in cost by about $5 some years (and then there's the holiday increase), so this encourages people to purchase dining plans. Then they typically don't care about the quality of the food relative to its price.
Smart move. It's allowed quality to go down and people to be even happier than before. Value and 'discounts' are what it's all about now.