Maturity, focus, concentration, self confidence all ebb and flow with time and aren't guaranteed to be monotonic. Civilization allows us to do great things, but it also enables the individual to be a weaker total person while being particularly skilled in one area.
Is that actually the case? I feel like a hunter/gatherer society isn't particularly well rounded - it's just a couple of skills that one lives or dies on. I wouldn't say that a caveman was well rounded, instead they were just ignorant of everything outside of "find something to eat and kill, then mate with something else".