The "do what makes sense" is limited because you have to ask questions to ascertain what makes sense. Some questions are easy to answer, but many questions can

The "do what makes sense" is limited because you have to ask questions to ascertain what makes sense. Some questions are easy to answer, but many questions can only be answered by doing a thing that takes work. And Scrum gives you a framework that tries to put a limit on how much work goes by with no answers. So it uses a simple schedule. The team works for X weeks, working on implementing specific tasks. (Which in turn answer some questions.) Then the team stops, reviews their progress on the larger product, and adjusts plans accordingly. Now "do what makes sense" has demonstrably advanced.