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?
Yes. That's how basically all expansion to the field of human knowledge is constructed today.
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
Some graduate students will likely spend a substantial part of their PhD understanding this paper. They will learn a lot in the process, and then they can contribute by either extending the result or finding a way to simplify a part of the proof. Or if they have a (very) related interest, they may be able to adapt some of the techniques to the problem they're interested in. Slowly over time, through this process, the knowledge might diffuse to other less related areas.
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.
[1] http://matt.might.net/
[2] http://matt.might.net/articles/phd-school-in-pictures/