That is not actually true, there is a lot of randomness, and in quantum mechanics the randomness is fundamental. The real secret of the natural sciences is that good physicists never publish if they know they don't know the answer. It took hundreds of years of that policy before the narrowly applicable theories blossomed into the general laws of the universe. If the focus was determining the general laws of the universe from the start no progress could have been made: Newton could have guessed at the laws of motion, but without the specific and painstaking celestial observations preceding him nobody would have known he was right. Economists could enjoy a similar success, but it would be at the expense of 400 years of saying only things that policymakers thought were obvious, while refusing to answer useful questions.
There are plenty of "very true" theories in economics, but they often aren't broad enough to answer the questions about economics that the fate of nations ride on. However 400 years from now those narrow but well-tested theories will be the foundation of the true general economics and all the rest will be forgotten.