Perhaps my self preservation instinct is completely broken, but why are people so afraid of the possibility that the human race could be replaced by hyperintell

Perhaps my self preservation instinct is completely broken, but why are people so afraid of the possibility that the human race could be replaced by hyperintelligent machines? We aren't perfect (quite the opposite), and a brain that works in the way that ours does has severe built-in limitations. Perhaps the greatest achievement the human race could ever obtain is to create something greater than ourselves. Something that can carry on learning and understanding the universe around us in ways that no human mind ever could.