Through my own experience hustling for clients and in coaching presentation skills among job hunters I know this: We have to invest time in doing the cover letter right. "Right" means creating a narrative that aligns with the poster's idea of the perfect professional to do that job. Of course, that requires analyzing the specifications to uncover that wish list. Then we have to show that we are indeed that pro.
The trick is to make a judgment call: Do I want such-and-such an opportunity enough to put in the legwork? Yesterday on Mediabistro.com there was a help-wanted ad which demanded doing plenty of homework and thinking. For example, we would have to surf the web to identify and find sites which were models of the kinds of writing the poster wanted. Then we would have had to provide quite a few topics for possible articles. The laundry list of requirements didn't end there.
A Millennial I coach asked me: Is this worth the time? There is no absolute answer to that. Everything depends on how much you want the opportunity. Also, you need a good scam-detector. The poster might just be fishing for ideas, free. If the name of the organization is given, the possibility that it's a scam is lower. If the whole enchilada seems to walk, talk and smell scammy, report the poster to the website.
In short, finding work takes work. You gotta get shrewd about which seeming opportunities to make it your business to showcase your edge.