It's not just fair use. Some people actually license music either direct from the musician or through a licensing service. Sometimes years later the video that

It's not just fair use. Some people actually license music either direct from the musician or through a licensing service. Sometimes years later the video that used the music gets a copyright claim from someone else besides the actual musicians that wrote, performed, recorded, and licensed the song. Sometimes it's about a different version by a cover band, sometimes it's a business that blindly uploads their entire catalog into ContentID, sometimes it's a scammer uploading literally everything to they can hoping to get free ad revenue from whatever ContentID randomly selects.