a big reason that they seem to be "ripped off" a lot.
Everything has been done when it comes to the actual accompaniment behind the songs. Changing up the instrumentation or rhythm might trick your ear to think it's unique but it's not. The melody is the only thing that is unique and when you think about the fact that there are only 7 unique notes in the scale of any given key, it's pretty amazing that there seem to be an unlimited number of combinations/patterns that are still quite unique.