Why would it be a bad idea to redraw borders to reflect population movements, shared values, and similar things? Consider Brexit. It's going to cause someone t

Why would it be a bad idea to redraw borders to reflect population movements, shared values, and similar things? Consider Brexit. It's going to cause someone to be very unhappy - either the rural hinterlands who don't want to be part of Europe, or the Londoners/Scotsmen who do. Why not let them both get what they want? (Obviously the benefits from boundaries that reflect shared values need to be weighed against the cost of the transition. I'm only suggesting it's far from clear that this is a bad idea.)