I think this really hits the point. Since the vast majority of these are no different from spam (in fact, I'm convinced some percentage may be from reengineered

I think this really hits the point. Since the vast majority of these are no different from spam (in fact, I'm convinced some percentage may be from reengineered spam software), it's those that actually respond that they have a hope of snagging. It's entirely possible that these recruiters are paid by how many actually respond, not how many are worth hiring. I.E. Spammers send thousands of emails and if even one sale was generated, then it was worth it as it costs them so little in the first place.