But if an estimate as to how often a pipeline segment will fail from a certain threat is needed, aren’t numbers needed to ascertain how often similar pipelines have failed in the past from that threat?

No, it’s not essential. It’s helpful to have such numbers, but not necessary and sometimes even counter-productive. Note that the historical numbers are often not very relevant to the future – how often do conditions and reactions to previous incidents remain so static that history can accurately predict the future? Sometimes, perhaps, but caution is warranted. With or without historical comparable data, the best way to predict future events is to understand and properly model the mechanisms that lead to the events.