[hnfong]

I agree most recommendation engines simply suck. (I can't imagine how they spent millions hiring Machine Learning PhDs and end up with that crap)

That said, youtube recommendation algorithms are usually sane. I suspect the depth and breadth of available videos (which are actually potentially interesting) helps, whereas for ads, the platform's primary goal is to shove unwanted stuff down viewer's throats, and if 99.9% of the people don't actually want to see the ads, they still need to pick the ones "most likely" wanting to see them, even if that probability is like 1%.