Well, DH and I are childless adults who really love Disney. I'm not going to get mad at the comment, but I will give an answer to "why would someone without kids go?"
Because it's magic. I love that it's an insulated bubble of happiness, good times, and family. I don't have children, and probably never will (by choice) but I LOVE seeing kids light up at a fireworks show. Heck, adults too. It's great to be surrounded by imagination and whimsy- which are normally completely stamped out of adulthood. We went with some friends a few years back and got to watch one of them see Fantastmic! for the first time. It was a heartwarming moment to know that adults can still have that shock, and awe, and surprise that you forget about as you get older.
My husband is a bigger Disney lover than I am. Always has been. But over the last few years, it's taken a different place for him. He works with emotionally disturbed kids for his job. Lots of mental illness, but also lots, and lots, and LOTS of severe abuse. Abandonment. Lots of kids that are shuffled through the foster system. Disney is a retreat for him from all of that. When we go, it's like taking a giant weight off his shoulders because he can just be the goofy guy he is, without a care in the world, gorging himself on popcorn. And not to sound totally creepy, but being surrounded by all of you parents and your kids brings a smile to his face too- because it's nice to be reminded that there are devoted parents who love their kids and share wonderful experiences with them.