I think this is a misconception.The thing that comes to mind for me is that, IMO, INTJ's take a more proactive approach to trying to prove A, B, or C. They are forging forward and discovering new truths. They have theory A and so they set out to prove it. If they are able to prove it, then maybe it helps society or helps the situation they are in.
With INTP's, there is truth and then there is everything else. It's more of a passive approach in that we see the truth as something that is out there to be explored. We're not trying to change the truth or forge into new territory necessarily - we just want to understand truth as it is.
One reason that I think you see more INTJ's in Ph.D. programs than you do INTP's (I have no "proof" to claim this as true, but I believe it is) is because the INTJ has no problem choosing a subject they want to study and then forging forward and doing research in that area. In the process of choosing something to study the INTJ has, in essence, forsaken all the other areas of study. They can still have outside interests, but the bulk of their time will be spent in their subject.
The INTP doesn't want to (or has a hard time) choosing just one subject to explore because he wants to explore truth as a "whole". If I go get a Ph.D. in Physics, then I feel like I'm abandoning the arts, World History, Psychology, Business, Nutrition and Physical Education, spirituality, and everything else that encompasses "truth". I want to explore it all as a whole. I'm not interested in picking one of those things and making it "my own personal expertise". I want to be knowledgeable on all of it.
INTJ wants to uncover new truths in a given area - proactively advancing what we know as truth.
INTP wants to understand truth. It is what it is, we don't want to change it or manipulate it. We see it as unchangeable, and so we just want to understand it in all it's complexity.