Good question though. Maybe it's a chicken and the egg sort of a thing? Like, what if the hair grows (retains length) because it's already healthy, so there's no real need? Maybe just minor maintenance (regular washing/conditioning) is all that it needs once it hits that equilibrium of "health"?
Also, long haired people are probably doing less manipulation, so less breakage. They've had time to perfect their bunning and protective styles, and have reached a level of already knowing how to prevent damage, so a DC isn't all that necessary? When my hair was short, I was doing all sorts of stuff to keep it presentable looking.
Then again, I'm just talking out of my @ss