To further qualify claims of plagiarism I only have anecdotal evidence.
There are minor issues such as incorrect or absent sourcing and footnote mistakes , then there is the wholesale Melania type copying. Add ghost writing and other issues like overseas translations all together and it is significant but never 'most'. There are probably fields where cheating is impossible.   
I have no aim to disrespect the accomplishment but when Human nature has a will there is a way. The number of people involved in the agonizing process prevents most wrongdoing.

Too bad executive office does not require advanced degrees or any degree for that matter.


First jobs like paperboy does stay with us. It does with me.