Disney always intended the dining plans to be "all or nothing" programs. Every day...every traveler.
If they allow any sort of flexibility, people will take advantage. It's just human nature. Sure there are people who legitimately have members of their travel party staying for just a portion of the reservation. But on many, many more occasions, guests would simply lie in order to get a small number of dining credits which best suit their desires.
Financially the dining plans work best over long vacations--say a week or more. At least from Disney's perspective.
When you check-in and are issued 7 nights worth of dining credits, chances are some of them will be used for restaurants or meals which do not completely maximize the value. Most people won't use credits for 7 dinner character buffets or the highest-price entree at non-signature restaurants. Over that many days, they'll end up eating at some cheaper locations or getting a sit-down breakfast or two.
Heck, many people even leave with credits unused.
But allow those same people to buy just 2 nights worth of credits during a 7-night stay and suddenly they're dining at the most expensive restaurants and allowing nothing to go unused.
With one set of credits on the standard
DDP, you could have dinner at Chef Mickey's, lunch at Pepper Market plus the refillable mug and snack. That's upward of $75 in value for a $55 price tag.
But Disney never intended for the DDP to be a 25% off discount program. Ideally, for every day when you get $75 worth of food for $55, there's another day when you're only consuming $35-45 worth of food for the same $55.