Well that's the thing, the easiest **** to upload (if you have the talent) is a song cover, i'm sure there are literally thousands of channels of ppl just uploaded cover after cover for a myriad of reasons.
1. good exposure.
2. to make money, you upload enough, you get enough unique views and you're asked to be a youtube partner and split the profits with youtube, and this is where the DMCA is pissed off, they don't want anyone making money off copyrighted songs.
You can't really say it's new material, just repurposed.
Hell, you still have to pay the *******s that own "happy birthday to you" if you sing it in any type of media, it is not a public domain works. Every family member that sings happy birthday is basically covering a song.
I'm sure if you made a documentary, and scooped up some old footage of happy birthday being sung, you'd have to pay the happy birthday company for rights.