I would assume that a piracy site might not be using Google's ad platform. YouTube also has a lot of recognition as a popular & legitimate platform, which likely seems juicier to advertisers compared to a generic site. (Those advertising on a pirate site might not know that the ads are shown there)

YouTube also is not in a big need of extra revenue, unlike some other, smaller sites. And, depending on the site, they may or may not be able to show big, unskippable, profitable video ads before access downloads.

Please support small independent sites with ad-views. ;)