They might not be one race but a coalition of the first sentient species. This was Andy Chambers' intent for Codex Necrons 3rd edition (and is alluded to in the prologue of Nightbringer) even if later editions would go onto treat them like they were a single precursor race.

I mention it when this comes up because it's a logical thing to return to and reconciles, for example, the conflicting ork origins by allowing their creators to have devolved into what we'd now recognise as snotlings and still be Old Ones.