You're right. Those are completely separate millions of dollars to be made. Best not let them overlap at all, even if some of us intuitively touch our laptop screens to scroll a window now and then, especially when carrying it around.
They overlap plenty where it makes sense. There was a time where Mac users were worried that Apple was going to merge MacOS and IpadOS. Nobody wants that.
What is that reason? Clearly Apple can put desktop chips in the iPad, their desktop apps already support ARM, and the iPad hardware can absolutely run macOS. What is the advantage of iPadOS, instantially?