You would think that they would understand that day-visitors and first-time visitors (who make up the bulk of single-day ticket buyers) are very important, and turn into regulars later if they enjoyed themselves.
But many (and I know some) simply won't pay that for admission, when they can just go to the beach with the family and have a great time for a small portion of the expense. They look at Disney, see the posted costs (or hear about them), and make their reservation for a week at Nags Head or Myrtle Beach.
Once people get in the door, they also eat, drink, and shop for souvenirs. But if they never get in the door, they never spend a dime, much less for souvenirs...
(And if they do get in the door, they find that they already have Mickey shirts just like the ones on sale, and no unique merchandise to buy -- but that is a different argument altogether!)