first of all, because the client made them and left them there (I'm not getting into his/her reasons).Why would you want them to be?
I knew u'd like this...I am sure the client would want their own personal backups to be transferred to the new server and not left behind on the old server.
I am actually glad this was brought out. I have been transferring hundreds of clients over the last few weeks. Now I have to go back and transfer all of their own backup files.