I worked in adtech detecting and blocking this stuff circa 2011. Back then it was common for sites like these to be used to launder impressions into higher-paying CPI categories. So you might have an arcade site (low CPI) include a hidden iframe for a fake beauty site.

It was honestly pretty easy to detect, but it seemed to me that the industry was just looking the other way because everyone in the industry was just trying to drive volume for their eventual IPO.

I think now the iframe stuff is harder to get away with, but botnets were a big source of traffic as well.