The lyrics & melody might be copyrighted, but guitar/bass riffs and drum patterns can't really be. There's a lot of old blues songs that have the same guitar riff, same for reggae. Some types of music just have a generic sound that makes it readily identifiable to what it is. For any type of music, there's only so many notes, so is anything really unique? Maybe someone can do it with some kinds of avant-garde music, but that generally doesn't appeal to a wide audience. There's a video on Youtube where some guys demonstrate that a lot of pop hits have the same 3 or 4 chords and so are interchangable with each other. The only difference is that the songs have different lyrics or different instruments. I think the video is used for humor, but it still shows that a lot of songs sound alike.