YouTube realised there were a lot of kids channels, tops 10s, compilations, music, etc etc so they decided to deny or remove monetisation from those channels by calling it Reused Content. Here's a better explanation from Longzijun:
2.3 Videos that have been done to death already. A good example of that would be ‘learn color’ and ‘baby shark song’ animations. There are tens of thousands of these videos on YouTube already. They do get a lot of views, but children really need to learn more than what the videos are offering. Channels specializing in these kinds of videos are not getting monetized. Another problem affecting these kinds of animated kids videos is that they are targeting toddlers while YouTube (and its advertisers) are targeting people 13 and over.
Source: longzijun.wordpress.com/2018/06/15/youtube-monetization-and-the-issue-of-duplication/
Yes I know, other channels are doing it! I guess big channels can keep doing this OR they will eventually be reviewed and YouTube will remove monetisation from their channels. They've done this with 100K+ and 1M+ subs channels so you can just wait and see.