Proofreading theses written by candidates for degrees in various fields furnished a goodly part of my income as an undergraduate student. My work went beyond the correction of spelling, grammar and syntax.
Quotations, clearly attributed and otherwise, had to be checked. More than once I found that the theses of two or more students on the same general subject were remarkably alike. And why not? They had dug up bones from the same academic graveyards for re-burial in new crypts. (That is an indirect quotation from someone whose name I've forgotten.)
I came to the jaundiced conclusion that such theses are usually regarded by the candidate as onerous chores, to be labored through with as little pain as possible. They serve mainly as proof that the students have covered the same sterile ground and suffered the same grind as their professors before them, and you may be sure that those professors, having themselves suffered, will see to that.
Plagiarism? Just try to say something that has not been said before. Dr. Martin Luther King is in very good company.