When jobs show up in those sites, the company already failed at filling the position through trusted references. (or so is my impression)
Doesn't have to be bad ofcourse, but propably warrants more due dilligence on the job seekers part than a good reference.
If you're looking for a job as a place to make friends and work with interesting people, fine, that's good stuff. But, many places of work are simply businesses, trying to make money for their owners. Recruiters want to find competent and experienced workers who can come in and get stuff done. It's not all lowballing and keyword bingo.
When we have openings they're usually reluctant to apply because the positions normally benefit from a solid math or sciences background, or they'd rather be working on more general-purpose software than that used for drug R&D.
That's mostly why we use job posting sites and we've actually managed to find solid team members via StackOverflow and some local job posting sites.
Getting a job via a job board and recruiter won't be all warm and fuzzy like meeting an employeer at a conference. It will be more like buying a used car, but you still don't have to apply in this market.