Have you ever tried to plan an invasion? Like, actually done a net assessment of a scenario in detail accounting for terrain, equipment, training, intelligence, logistics, and all the other things that go into a military encounter? It is time-consuming and hard. So is efficiently butchering a hog. So is learning to program well.

Changing diapers and pitching manure? Not so much.

Dying gallantly? Only get to practice that once, so how would you get good at it?

People throw this quote around because it is fun to imagine yourself as a supremely Competent Man in all these domains. Indeed, it makes for excellent fiction—-but it is just that, fiction. When you actually start trying to seriously think about investing in specialized skills like military science, this quote quickly reveals itself to have not made any distinction between life skills that the average person should have and things which require years of study.