Typically, PhD theses are expected to have novel results. As science advances, it is becoming increasingly difficult to accumulate enough knowledge to begin a project that pushes the boundaries. Original findings are often published as papers before the defense, thus the thesis has become valued for its indication of technical expertise, not the discovery itself.
Since ancient history, redoing the work of masters has been seen as a very effective tool for training. It so happens that there is also a need for reproduction studies, but few are willing to do them.
Why don't PhD theses consist of reproducing important, recent, controversial research? Has anyone tried this?
Note: I am deliberately leaving open to interpretation whether the student would reproduce their own advisor's work or that of a researcher from a different group, and whether the original researcher is to assist in this and to what extent.