Why do so few characters use JCMs
Character morphs are quite limited by the weight map if JCMs are not used.
It can take me well over 20 sculpt revisions to get a character morph right. But the 40 or so JCMs take less than 5 hours to sculpt and setup. Seems like a small time premium considering the benefits.

Comments
I find it quite impressive if you can fully sculpt and rig 40 JCMs in 5 hours. Not saying I don't believe it, just that I'm impressed. When I am working on clothing, it can take me days to get all the JCMs right for just one article of clothing.
The JCM sculpts are reasonably very similar for characters, I guess you get an eye for it.
Or more precisely, the problem rotations are very similar for characters (Chest, Shoulder, Collar, Thigh, Shin, Forearm, Pelvis)