Why do jobs go through a staffing agency?

Is it really that hard to throw up an ad on Craigslist and set aside 1 hour for interviews on a wednesday afternoon?

My best guess is because of liability. If employers go through an agency, that means they can get an employee who's not technically their employee (unemployment, work comp, wrongful termination makes the agency itself liable.)

So my question is: Is the only reason employers go through agencies is to shift liability, or am I missing something here?

I can through up an add on indeed for coders, and get 100 resumes by sunset. It's not hard finding employees in this market.

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