It's not as silly as you might think
If their tracking methods rely on people clicking through to the rating site, then the sites that want to be ranked encourage users to do so.
For example "Please click the SUPERMEGARANK banner at the bottom to make us number one!"
When the user clicks through, not only do they see the rankings for the sites they also see banners that are paid for. The rankings site then gets money for showing banners, but only has to do the initial site setup. The sites that want to be ranked do all the work trying to get traffic to them.
Back when banner advertising was worth 2 or 3 dollars per thousand clicks, some people made a killing. Others paid off their tuitions