Because the 500 acres of Disneyland has the same type of road, water, sewer, electric, transit, fire, ems issues when compared to the 27,000 acres of Disney World?
There's no obvious difference between the two that creates differences in how those problems might be solved and managed?
Is there anther similar attraction in the US that cover more than 25,000 acres? How about 20,000, 10,000, or even over 5,000?
Not like a park where it's mostly open space. But, some other attraction that has a similar size of building infrastructure even if not the same number of visitors, but say at least 50% of the same number of visitors. Before someone suggests the Grand Canyon or some private hunting club on 50,000 acres of woods with 10,000 visitors a year.
It's not that you couldn't have WDW without RCID. It's that having it has created better ways to evolve it than not having it would have, at the scale of WDW.