I have a suspicion that the reason universities crack down on plagiarism this hard (to the point of outright making up offenses like ‘self plagiarism’), is that it’s the only form scientific misconduct that is easy to prove and investigate.
If you are wondering if it’s true, just look at how long it took for Hendrik Schon to get caught. And even then, the smoking gun was reusing (fake) graphs in a publication.