I think of it this way - pretend you're designing a new body for a 1/6th figure and you want it to have all the articulation needed to get into any position a human body can. It also has to look natural under clothes and every pose you put it in has to conform to how human bone and muscle and skin would look in the same position.
So you look at all the bodies that exist now - buck, dragon, HT, medicom, volks, etc. and start designing your body...
The midsection needs to turn such a way and needs to be segmented just so. Does the buck do that? no. HT? no. dragon? no. medicom? almost, but not quite...volks? yea, that's the best solution to the midsection problem.
Okay, lets move to the thighs. They need to bend certain ways but not leave unnatural looking gaps at the hips. Which body does that? Dragon? no. HT? not quite. Medicom? nope. Volks? hmm, yea that's the best solution to that problem as well. And Takara used the same style hip/thigh join for the Cool Girls 2.0 - I'm not sure if it was on the Volks or the CG first.
But so moving on - looking at every area of the body, you look for the best solution. Some places there was a better way of doing it than on the Volks, so that solution was used. You take the best of what exists and put it all together to deliver a state of the art body. Why re-invent the wheel?