Deliberately shaping public opinion in unpopular directions therefore seems like the most likely motivation.
The other, slightly less nefarious possibility is that they don’t actually want us to watch videos and drive up bandwidth costs. Good engagement means people just sit and watch the video instead of clicking something else and be served another ad. When people clicked a video and saw that it was overwhelmingly disliked, they would leave immediately, but now they are forced to sit through the whole ad to see what the video is about.
It’s also possible that people with good taste are more likely to be using ad blockers, so a content connoisseur is really just a parasite in the eyes of Google.
I think you are also overestimating how much people look at downvotes. Most people click, watch and move on if it's not good. Have you ever seen a Tiktok user?