Doing so would be a mistake so severe it would damage sales of both iPad and Mac. It would take years to clean up the mess.
No matter that Macs and iPads run on the same chip. macOS isn’t designed for a touchscreen. Apple would need to redesign the operating system to run on a tablet, and devs would need to alter third-party applications, too. And that would anger people using non-touchscreen laptops and desktops.
Plus, people happy with the iPad now wouldn’t like the change either.