I’m fairly certain I read somewhere that During construction of WDW, that WDI had to take over for the general contractors of both the Magic Kingdom and the resorts in order to open on time. Disney’s not in a position to “resume” that level of involvement. The cost and staff are both prohibitive. Furthermore, that amount of in house staff requires a pipeline of new work. Otherwise, there is going to be a lot of hiring and firing. WDI has more or less become a construction manager or owners representative.
The typical 500 room hotel in North America takes 18-24 months for construction. So it’s not a surprise that any of these attractions or projects take the same or considerably longer.