Perhaps they traced back to Adam and for lack of anymore 'hard' history, decided to begin with Adam. Makes life simple!
“decided to begin with Adam, for lack of anymore ‘hard history.”
This explanation would satisfy the question of how Cain left the family and found his wife (unrelated to Adam and Eve).
Well it was not a they that did the 'tracing', it was by design, because of what had already took place in the previous heaven/earth age. It is written that the remembrance of what took place back then is not given to us as we pass through this flesh age. However, the evidence abounds there was 'life' upon this earth prior to this flesh age that humans walk in flesh bodies.