because quite obviously it doesn't......
as you yourself admit, it happens within a smaller segment that is isolated from the entire population......it changes from what the entire population was and becomes what the entire population is not.....and that does not happen identically in multiple locations.....different things happen within different areas of isolation......
humans dispersed from the area of isolation in which they existed....they did not simultaneously appear in multiple locations.....and they took their domesticated animals along with them.....
now, can you give me a true example of multi-regional evolution........
perhaps a unique form of beetle found in both Africa and South America?.....