Because it's their plan, and they can change it at any time. They will change it when they hit some road bumper. They will pause it if they face some resistance. Most times they will not pick it up again, and they will create a new plan. This actually happens to me multiple times.
So if you want a plan, it's better to use other people's plan. Use some plan that you have no right to modify, to pause, or to cancel. That's why I created a plan for 5334 course, so all my students could follow that plan. Nobody could change it, except me. We get on the 2-year project, and we will finish it.