They did not implement a winner-take-all format. They said that each state legislature got to decide how to pick electors. Why so many states have gone to winner-take-all is probably expediency. Nebraska, of course, proves that the constitution does not require winner-take-all.
You’re right there. But the Framers in their foresight should have seen what would happen and forbade winner-takes-all in the Constitution. Would have saved us a lot of grief. But the Framers weren’t perfect. There’s a lot they didn’t see coming. The book “Uncle Tom’s Cabin” and the place names Gettysburg and Antietam come to mind. Talk about kicking the can down the road!