They hire people in 3rd world countries and say something like "I want 10mil uniques to this tube this month and I'll pay you $500 to make that happen. I want it done no matter what so if anything is done illegally, I don't want to know about it... If you succeed, I might hire you again next month."
Also, tube sites naturally will have a lower bounce rate than most websites, and that helps too.
There are a lot of reasons for it that basic SEO can't explain.
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