Because the world is messy
We needed a method to determine which statements about the world were ones we could all agree were, indeed, facts of the matter. And the essence of the method we came up with, the one called science, hinged on something absolutely remarkable in the history of humanity.
It all depended on an agreement.
Over time, and as a society, we decided to agree what the rules of the fact-finding method called science should look like. It went something like this: Public facts will be accepted as public facts, if and only if you can show multiple and independent lines of public evidence to support them.
So writes Adam Frank in a very good blog post on how science is the method our modern society agreed upon to deal with the messiness of the world. He cautions that our use of this method is based merely upon social agreement, and that can be broken. But, he wonders, is there another method? For the sake of our democracy, we should be concerned.