WDW doesn't require a passport, or even leaving the same time zone for many of its guests. Some may even live within a day's drive, or less, or qualify for state resident discounts.
Many Americans already travel to Florida to board a cruise, attend a work conference, visit relatives or some other reason like escaping colder weather back home. Adding on WDW to that trip is a natural choice for families with young children who know other families who go.
Having said all that, if you're a big enough Disney fan to go that often, can afford to stay Deluxe, or own a DVC contract, it's worth it to visit TDR, and Japan as a whole. Same with DLP and HKDL IMO.