MacBook Pro internal display resolution wrong
This is the correct behavior. The actual resolution of the panel is 2880 x 1800, but macOS scales the UI to simulate 1440 x 900 @ 2x by default.

You can change the scaling factor in Display preferences to "create" more screen space. If you set it to the highest setting, 1920 x 1200, then you'll be able to drag a window from the external to the internal display in the way you're expecting.