However, isn't this also the consequence of various SEO tricks? A website such as YouTube can do SEO way more efficiently than someone who produces their content manually. If you have billion pages of content, you can easily make billion pages link at whatever you want... perhaps the things where the users most often click the ads, or whatever.

Basically:

Google: "good pages are those that have lots of links pointing towards them".

YouTube: "let's make all our pages link to the 'why quantum mechanics proves Trump wrong' viral video, people seem unable to resist clicking on it when they see it among the search results".

Random guy on internet: "I wonder what quantum mechanics is... let's ask Google..."