Over a long enough period of time, these divergences will become so great that member from different isolate communities will no longer be able to produce healthy offspring. This is how we get different species of relatively similar animals (sun bears vs polar bears for example).
Human communities have never been isolate for so long that different species of humans would form, but there have been sizeable enough periods of isolation that smaller changes have formed among different communities (i.e. different skin color, different tolerances to various substances (like alcohol), differently shaped facial features, etc).
The reason you see this more in humans over other species is because humans are so good at surviving that we’ve populated almost every corner of dry land (and its been that way since back in our hunter-gatherer days). Due to the advent of farming, however, individuals stopped roaming very far from where they were born and, due to geographical restriction (Sahara Desert, Atlantic Ocean, Himalayan Mountains, etc), and only reproduced offspring with some communities to the exclusion of others.