I think this behavior is probably driven by people like me, who use youtube mostly for music. I repeatedly listen to/watch performances on youtube because to me it's the only streaming service I have. I frequently come to youtube just to watch stuff I've already seen. I find the recommendation in that context to be uncannily good and they keep me coming back to find new music and to see the cool performances I've already seen. Maybe youtube has trouble differentiating these use cases?

When I'm programming, gaming I mostly listen to retrosynth and outrun.

For that youtube is great because it's autoplay just seamlessly plays one after another.

For everything else it's recommendations are hilariously bad.

It's possible your adblocker is preventing YouTube from tracking your watched video history. I know pi-hole does this by default unless you whitelist certain domains.

user agent hits Google random-string.googlevideo.com/videoplayback with referrer, ip, signature and several other parameters coding your individual user id, Google knows every single time who, why and when requested particular resource.

Just because they have the data doesn't necessarily mean that they use it. They might rely on some javascript XHR to tell them when a video is really watched. Just guessing here.

Happens to me on non-adblocked mobile all the time, it is infuriatingly dumb. Google seems to not be able to do user friendly UI's.

Don't get a started on discovery!

I think when you turn off tracking what you watch it warns you this will happen. it does this for me i think and didnt before i turned off watch history.. but maybe i am just imagining that. And maybe you havent turned off watch history.

I wondered how many others experienced this. It really makes it demotivating to use YT

What drives me insane is there's no way to mark videos as watched. You can hide them only with "not interested" even though you totally are.

When I mark a video as "Not interested" I get a link to "tell us why", where "I've already watched the video" is an option.

I wrote my own tempermonkey userscript just for that occasion to hide already watched recommendations :/