some tend to plagiarise because they are dumb and they can't produce their own pieces. :D

kidding aside, maybe because they want to receive some admirations with regards to the work that should be given to the owner of the piece.

or, they simply need the same piece done and they are just tired of producing their own. 

but then, there's a law that plagiarism is a criminal act. So, if someone's plagiarising, sue him! :D