A lot of people wonder why we study and document things that are already “common knowledge”. This is true of scientific studies as well. What a waste of money, right?
The answer is, until you actually do the work you don’t actually know. Scientists and government officials can’t cite common knowledge. And even if you were right about the conclusion, the details matter. The amount matters. The mechanisms matter.
High-quality studies also lay out a methodology for evaluating, assessing, and ultimately characterizing the issue, so that the impact of policy changes can be properly assessed. Even showing that well-known investigatory methods function adequately for a given problem is of value.
Put another way, "you can't control what you can't measure" (or in this case, characterize more generally).
The answer is, until you actually do the work you don’t actually know. Scientists and government officials can’t cite common knowledge. And even if you were right about the conclusion, the details matter. The amount matters. The mechanisms matter.