They don't use fake addresses to be untraceable. All spam ultimately tries to sell you something, so it has contat information of some kind in it somewhere. However, spammers know that many of the millions of e-mails they send will bounce, and they don't want the bounces. Also, the vast majority of the population still hits reply and sends a response like "take me off this list you bastard!" when they get spam. (I know this because a spammer once used an address of mine as a fake return address, and I received all of them.) The spammers have no interest in getting thousands of those either.