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.