Why would I want to do that?  I received the money in a legitimate transaction, presumably in exchange for something else that I no longer have.  Why would I turn around and give it to someone else?  Exchanges of currency in real life don't work like that.  Odds are that one of the bills in my wallet was stolen from someone at some point, but that doesn't mean that, if I could track down the original person it was stolen from, I should give it to that person.  I received these bills from bank, which gave them to me because my job deposits money into said bank.  I don't owe them to anyone else.  Likewise for Bitcoins.