From the linked Variety article, "However, over time, Netflix realized that explicit star ratings were less relevant than other signals. Users would rate documentaries with 5 stars, and silly movies with just 3 stars, but still watch silly movies more often than those high-rated documentaries."

But the signal isn't just for Netflix; it is also for users, who might sometimes be in the mood for something silly and sometimes in the mood for something good. Also, people might rather get more suggestions of good movies even if they are more likely to watch bad ones. (Of course, people might also just overrate documentaries.)