[loa_in_]

It's not Google per se but a practical impossibility: it has to rank somehow, hopefully as a human knowing the answer would. They could theoretically hire humans to do it but they won't because it's obviously impossible due to how vast the dataset is, so they use software. Software is still far from human level reasoning so they use metrics. Metrics can and will be discovered and gamed, regardless of what kind they are.

[prawn]

As an AdSense user, if you don't use the maximum allowable number of ads (regardless of content length), Google literally emails you to suggest you add more ads. Their documentation encourages you to maintain a reasonable ratio of ads to content at risk of being shutdown, which pushes out page length. They push for unique content (so writers differentiate with personal stories), they measure time on page (longer details, pictures), etc.