We share that view of human nature.
In the case of these groups, however, the populations were so small and isolated that the groups remained largely distinct. It was only in the few instances where they came into contact with one another that there was mixing.
Also, it took a long time for the EEF and the WHG to admix, a thousand years or more if I remember correctly. That may have had a lot to do with very different life styles.
Religion also keeps people apart, i.e. Druze, Christian Arabs, Assyrian Christians in the Middle East.
I've always been a bit surprised how separate the German communities in Eastern Europe remained.
Perhaps the most mixing occurred with predominately male migrations or invasions with the local women, for obvious reasons.