1. Actively supplying negative feedback is sometimes hidden behind secondary menus, making it much higher friction compared to just...scrolling past. So most users don't spend the effort. Even with a dislike button, it's unclear what the system is learning. It can't know that I don't like this particular video because it's a conspiracy theory, and to stop showing me those. These platforms often don't even support explicit categories, so how would they know?
2. It's extremely high friction to teach the algorithm you're interested in something that it doesn't suggest to you! There's the whole unknown unknowns problem: how do you teach the algorithm you're interested in something that you've never seen before?
I still think Reddit has handled this the best. No system is perfect, but Reddit's challenges are much more manageable than the quagmire that TikTok, Facebook, and YouTube have gotten themselves into. I can just unsubscribe from r/conspiracy, and I'm out. Basically impossible to teach that to YouTube without weeks of careful curation. They think they're smart enough to know what I like, but they're not and never will be.
For example, I might see a some specific word or pattern of words in a tweet and quickly skip to the next one. That's very low friction but also a powerful signal. There are drawbacks (e.g. the long boring video that looks like something is about to happen but never does), but that's mitigated by combining this with other signals.
With 2, it's an explore/exploit trade-off. You try to explore as far and wide as you can while trying to avoid the things the user may dislike, all while sprinkling in just enough of the stuff that you know they'll like.
Yes, there are signals in human behavior that you can feed to your model. But no, it is never going to learn "on Mondays he works on his comics, so he'd prefer to see webcomic-related content" Don't sprinkle in your explore/exploit experiments. I know what I want, just let me decide based on what I'm in the mood for!
TikTok-style feeds are the absolute worst offender here, where they couldn't care less about what you think. They will serve you content, and you will either say "yes" or "no". So the only option for you as the user is to just wander through their content hyperspace. There's no structured way to jump between topics because everything lives in this formless content soup.
The other problem is that many social media platforms have you subscribe to the content streams of individuals directly. Individuals are high variance. How can you teach these engines that "I only care about this person's posts about pianos, not their terrarium hobby."
I pay a lot of attention to classes of things I don't want to see at all.
They just don't want to.
People already use hashtags, what would happen if you could subscribe to them? It would create a whole new way of interacting with the platform, something closer to Reddit where people post to streams. But the fact that these streams can't be moderated (or even downvoted) might be a nonstarter.