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

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.