This is because most if not all the jobs come from the companies website. They are a search middleman similar to how Google is for almost everything else. They don't provide an ecosystem like say HN, Stackoverflow, Yelp, etc. In some ways they steal the content.
Sites like Indeed are basically doing the same thing that Google really should have been doing.
Now the boards/aggregators tried to provide "ecosystems" but they could never get it right because when people are looking for the job they are desperate and just don't give a... (or at least that is my theory. its also why linkedin works because its is passive).
I have noticed through our metrics (and I'm biased because we make job web sites for companies that hire) that the best candidates (despite what Indeed says) come from going directly to your website or come from LinkedIn (assuming non referral based).
If you are wondering how then Indeed/Career Builder et al do so well its that its hard to measure actual success (ie source to hire). It should be easy but sadly its not (don't have time for the details).
So what these guys provide is ton of traffic with lots of noise. Traffic that google could have easily provided.
So honestly I think it is sort of good they fail.