Preview is awesome! My favorite feature is splitting up and combining PDF's by dragging and dropping the pages.
I have to do a lot of PDF manipulation for my work, and after some time I accepted, grudgingly, that there's no substitute for Acrobat Pro.
Why do they do this? It's not good for anyone. I can understand this concept of "just do what's intuitive - try it!", oh what fun - but nobody is going to risk messing up the important files they're working with by randomly dragging things onto other things just to see what happens.
Just tell us how to use it.
Almost every (third-party) iOS app or game by comparison seems to have understood this basic concept of educating the user on how to use the product, but Apple falls far behind especially on macOS.