Part of the problem is that it costs Steem Power to flag someone, so when a post contains a lot of comments that are spam, some of those comments end up staying visible because people don't want to waste all their daily power allowance flagging. The spammers know this by now and what I've seen is an increase in spam because of it. For the spammer, it's all a numbers game. For x-amount of spam posted, they'll get x-percentage positive response. Do it enough and the benefits to the spammer start to add up.
And when other people who are new or just struggling on here see that, it encourages them to try it themselves. Eventually though, spam makes a platform like this an eyesore to people thinking about joining. I'm sure you've seen forums that have been overrun with spam end up like ghost towns. That's where Steemit is heading if the problem isn't nipped in the bud soon.