Well the ‘humans’ on ‘earth’ were Cylon ‘skin jobs’, but the ‘Centurions’ on ‘Earth’ were not related to the ‘Centurions’ on the worlds of Kobol. Go figure!
Or the human-humans and humanoid cylons of Kobol could have fought a war with the cylon-centurians. Then with a devastated Kobol, they split up, with the human cylons not welcome with the “pure” humans.
Fast forward a couple thousand years. The Kobol skin jobs created centurions, fought a war, and all died. The pure humans, being inferior, required 2000 extra years to create their own cylons - who created skin jobs. Then the pure human colonies fight a war with their centurians and lose.
So whenever you have “skin jobs”, they make centurians and eventually get killed by them.
Cavil - who is a machine and embraces such - might be the last remaining form of life. He could make more cylons, begin a download process, and keep up the machine race forever. Skin jobs who are confident in being machines are in charge, with mechanical centurions kept dumb. As long as none of the “dumb” ones get smart and rebel ...
Lesson learned - don’t create sentient machines.