The current Internet email protocols have their roots in the 1970's. It was never anticipated that it would become the mass worldwide message system it currently is. Security was an add-on and initially thought not very important. Effective security in anything is not an add-on but something that must be designed in from the very beginning.
There have been numerous proposals for a more secure email system but obtaining worldwide agreement and acceptance, to say nothing of implementation, would be a massive undertaking. And there is always the concern, would it be truly effective?
Spammers go to great lengths to hide the source of SPAM messages, that being one of it's defining characteristics. If SPAM could be effectively traced it wouldn't be a viable form of advertising. Current email protocols make hiding the source a relatively simple matter.