They are. We currently get about 30-40 spam submissions a day. Turn on showdead in your profile and you'll see it all. The reason we don't get more is that we're very aggressive about killing spams. Most spammers give up eventually when they realize that submitting here generates near zero traffic.
Some spam gets killed automatically. Some gets flagged either by filters or by users (there is a flag button on stories after you get over a certain karma), and killed manually by editors. It's very rare now for a spam not to at least get flagged.
I think there are algorithms in place that auto-flag spammy submissions to bring them to the attention of moderators more quickly, but as far as I know, actual deletions are manual.
