Because you're spamming jobs in the same way they're spamming applicants. Why do the extra work?
Seriously though, I think this is false logic by the recruiters. Most people will go through the channel that's open to them, where they have been contacted. They're not going to try and find the right contact at the company. There's a tradeoff between people going around you and people ignoring you. I think ignoring is a bigger problem. A job that consists of keywords for an unspecified company with an unspecified salary is just not noticeable.
Seriously though, I think this is false logic by the recruiters. Most people will go through the channel that's open to them, where they have been contacted. They're not going to try and find the right contact at the company. There's a tradeoff between people going around you and people ignoring you. I think ignoring is a bigger problem. A job that consists of keywords for an unspecified company with an unspecified salary is just not noticeable.