It seems pretty straight forward to be honest, it goes back to "This has all happened before and it will all happen again".
It all comes down to how you define a Cylon, I'd say everyone in the fleet is a Cylon to some degree. 'Humans' in their base form, may not have not existed for hundreds of thousands of years. What we're seeing is that the Colonials have somehow forgotten their origins (originally created by humans long ways back), think they are 'human' (and what is human? created by god? lol) and have then gone on to create their own Cylons. Thus the whole exodus keeps repeating itself.
Maybe these 'final five' are somehow special in that they can retrace their roots, or have some memory of their true origins?
Cylons originally genetically engineered humans designed for space travel from earth to kobol? What kind of attributes would make a multi year space journey easy for a human. All depends on when and with what tech humans left earth... but its believable we will have huge strides in genetics before propulsion in the foreseeable future. How about a 22nd century earth faced with ecological devastation run by a faschist regime brought to power by said eco-social breakdown. Would they shy from eugenics or any genetic manipulation meant to help humanity survive an earth bound hecatomb? Hawkings warning comes to mind and would moreso under a bad scenario... If this is the world ai was originally brought into (assuming cylons were made by us earthlings) maybe they deemed it necessary to run us like gods vs openly enlightened advanced beings wanting to introduce us to their higher levels of tech and knowledge. Basically they dont trust us but dont want to see us dissapear as a species so repetive efforts by cylon cabal to inseminate their dna with ours? Who knows how long ago the 22 nd century is to them and how many and what kinds of attempts were made to save humanity from itself...