Primarily because the quicker you build something the more money it costs. In addition to that, rides are enormously complicated to build and get working - requiring dozens of systems and structures to wrap around each other and work simultaneously without killing anyone. Not only that, but say, compared to you average office building, there's a hell of a lot more artistry and craftsmanship that goes into building and creating the facades, rockwork, props, sets, etc. it's everything you need for a normal building (steel, drywall, concrete, plumbing, HVAC) plus everything you need for a couple dozen movie sets, except built to last for 50 years instead of 2 months. Pretty much everything has to be custom made and there's only so many people skilled in those disciplines. Finally from an operations perspective you want the attraction to generate the most crowds you can and not canibalize some other attractions opening, or interfere with other plans.
