Companies won't give you the 20% pay bump if you approached them yourself.

Its a good deal for the company because they outsource the risk of hiring a recruiter fulltime and not finding a candidate.

Some recruiting agencies make a lot of money but many go under. Companies pay a recruiting agency only if the agency found them the candidate. With lots of agencies competing, the fee is not certain. The good sourcers/recruiters definitely find it more profitable to work on their own.