Because the ultimate law of the universe is probability balance, both pure capitalism and pure socialism are terrible economic models. They both undermine real competition from opposite ends.
With pure capitalism the one to get on top first tries to suppress all below it by any means possible, not to mention the fact that out of control greed inevitably runs nations into a ditch. And of course pure socialism gums up incentive with bureaucratic red tape.
Countries with the highest standard of living across the social spectrum have a near perfect balance of socialism and capitalism, which in reality reflects the two sides of our human nature — competition and cooperation.