That is an ahistorical view of the history of patents. Openness had never even occurred to anybody when patents were originally invented. Back then, it didn’t matter. Humanity hadn’t come up with much that you couldn’t figure out how it worked if you had one in your hands. It may have taken millennia to invent movable type, for example, but somebody who saw it could have copied it immediately. Its relatively recent that that has not been the case for almost anything.

It was developed to spur innovation, and that is still its main function.