[Question] Why No Automated Plagerism Detection For Past Papers?

Automated plagerism detection software is common. But cases like the recent incident with Harvard administrator Gay have shown that egregious cases of plagerism are still being uncovered. Why would this be the case? Is it really so hard to run plagerism checks for every paper on Sci-hub? Has anyone tried?

I am curious since I am currently upskilling for the purposes of technical alignment research and it seems like an interesting project to pursue.