You people give out your email to anyone, don't you.
My *real* email address gets maybe 5 spam messages a year or so, and those are sent to an alias that's harvested from the publically viewable whois information for my domains.
Unfortunately, even if YOU never give out your email address to anyone other than friends and family, it doesn't mean you won't end up on the spammer's lists -- whois info can sometimes open the floodgates as well. As do those nice 'free' greetingcards sites your aunt keeps sending you stuff through that harvest your address in the process. As do those random other sites with cute little 'forward this to a friend!' links.
and once you're on one spammer's list, it doesn't take long before you're on *every* spammer's list. There's money to be made selling address, after all.
The only way to truly prevent spam is to not use email at all. :-/