(1) Because you have to pay the equivalent of 2-3 meals onsite every day of your trio when you might otherwise dine off property one day or buy groceries
(2) it discourages guests from going to universal/sea world/beach as a day trip because they are locked in to meals, so they pay for a longer disney park ticket
(3) keeps people at the parks or resort hotels where they may spend $ on other things like gift shops
(4) incentivizes Disney hotel stays for people who like an all inclusive feel (not available at Swolphin or good neighbor hotels)
(5) once you are at a disney restaurant you may buy extra stuff not included on the plan like a second alcoholic drink, appetizer (if not on deluxe plan) or souvenir tinkerbell/buzz lightyear cup for the kiddos (guilty of all these things)