Interesting, according to this blog post: https://blog.youtube/news-and-events/better-protecting-kids-...

> YouTube now treats personal information from anyone watching children’s content on the platform as coming from a child, regardless of the age of the user. This means that on videos made for kids, we limit data collection and use, and as a result, we need to restrict or disable some product features. For example, we no longer serve personalized ads on this content or support features such as comments, live chat, notification bell, stories, save to playlist, and others.

So it's to keep kid behavior from being tracked. Kinda cool.