Where there is money, expect scammers to lurk around, too. Bitcoin is value. In this forum you can make lots of money through different deals, trades, services and carrying out tasks. So I am not surprised that we have so many scammers here, and it is not just Bitcointalk. They are in other forums too, and it's even worse in some. At least here in Bitcointalk we do have the default trust and flag system.

I don't think there is always someone introducing new scammers here. Most of them are probably old scammers who have been busted, and they just create new accounts.