I think Benson's got a good concept, and I also wonder how much of it is due to the European tradition (2000 years plus) that all boats had keels... Did Native American canoes & kayaks have keels & rudders? I haven't seen many myself, but I don't recall seeing keels or rudders. So when European descendants started building canoes, keels may have "seemed missing," and thus got added, whether they were needed or not.
Or was the keel added as part of sailing rigs, to keep boats going straight?
Just speculating, on all counts... I have absolutely no information on which this is based.