The big companies post all of their jobs on their website, but the smaller companies definitely do not. In developing countries it is even worse. In India it is common for jobs to be posted on a website as PDFs, which are scan/images of printed pages.
There is added value. Indeed, and plenty of other job sites, have done things like quick apply which bypasses the horrific process of legacy ATSs like Oracle's Taleo.
People, in the most general sense, should be extremely concerned if Google -- or any specific company -- gains a market hold on job search / recruiting.
If, say Google and Facebook, or Google and LinkedIn had control of 90% of the hiring vertical they could, in effect, take a large chunk of an employee earnings. How? Employers would have to spend a lot of money just for the employee to know some opportunity exists for them. Big, high margin, corporations would be just fine. Small businesses would be disadvantaged.
This is what has happened with the travel aggregators. Two companies dominate (and own all of the other travel search engines you think are independent.) The big hotel chains can negotiate slightly better deals, the small ones pay full price for visibility.
In a more narrow definition we can see this with on demand work start ups. Uber, Postmates, and others have spend massive amounts of money to recruit workers. Of course as start ups they have time constraints and geography constraints. They need to grow really fast so they pay tens of millions of dollars on buying search ads, display ads, and so on. Perhaps they could grow organically but it would be hard.
A good job search / recommendation would just show users the best economic opportunities for them at this moment. Yes, it would be hard to rank and filter out stuff that looks good but is crap (Maybe Uber falls in the crap category?) However, if someone wants to work, certainly they shouldn't be sitting at home idle because a potential employer didn't spend millions of dollars on an ad campaign. (Those "reach" dollars should be going to workers, not platforms.)