That's because economic principles aren't scientific theories, they are political opinions (and weapons). That's why you have left-wing and right-wing economist

That's because economic principles aren't scientific theories, they are political opinions (and weapons). That's why you have left-wing and right-wing economists, and a broad range of theories on how things work, but no agreement on the premises. There's also the fact that real-world economy is a much more complex beast than any existing models precisely because there are so many variables. Which is why most economical "predictions" are just post-hoc justifications.