Can anyone build on results that are so hard-won and complex that understanding them is as much effort as learning the basics of some entire fields of study?
A human can only ingest and understand so much information in so much time. As Matt Might[1] eloquently described in "The Illustrated Guide to a PhD" [2], learning the basics of an entire field of study is what a bachelor's degree is for, a master's degree gives you a specialty, graduate students reading research papers like this one is how you get to the edge of human knowledge...only then can you start building on that sum of knowledge.
To summarize, a PhD often (maybe even typically) does not bring someone to the edge of human knowledge. Often a PhD gets people near it, but given the shear amount of scientific literature out there, it's difficult to know where the edge is.
I believe more than a few scientific PhDs do as well, and more than a few can become journal articles, or amount to a compilation of previously successfully accepted journal articles.
This item, an outstanding PhD thesis of an outstanding historian.
Free Soil, Free Labor, Free Men: The Ideology of the Republican Party before the Civil War
Eric Foner
It is akin to refactoring a codebase after it started as a spaghetti code behemoth.