Spotify's recommendation system is dealing mostly with artists that have recording contracts and professional production- their problem shouldn't be compared to YouTube's which has to deal with a mix of professional, semi-pro, and amateur created content. Also there's more of a "freshness" aspect to a lot of YT videos that isn't quite the same as what Spotify has to deal with (pop songs are usually good for a few months, but many vlogs can be stale after a week). Not only that, but many channels have a mix of content, some that goes stale quickly and some that is still relevant after many months- how does a recommendation engine figure that out?
It's better to compare Spotify's recommendations to Netflix's recommendations, which also deals with mostly professional content. Those two systems have comparable performance in my opinion.
Why the content exists is also important. People create video specifically for Youtube. Very few people create music just to host it on Spotify. This results in the the recommendation algorithm and all its quirks have a much bigger impact on the content of Youtube than Spotify. Also having that many people actively trying to game the recommendation algorithm can pervert that algorithm. That simply isn't a problem for sites like Spotify or Netflix.
It's better to compare Spotify's recommendations to Netflix's recommendations, which also deals with mostly professional content. Those two systems have comparable performance in my opinion.