Comment by parable

Please forgive my stupidity, but is there any particular reason why companies never seem to disclose what "system" of theirs was breached? If this breach came about as a result of a security incident at a third-party, would it not benefit everyone minus the vendor to shift the blame? I understand that they say "our systems", but I've seen companies say this before when what they mean is "a system controlled by a vendor we contracted".