But it isn't necessarily true. I just went the rounds of interviews for software development jobs and the recruiters were definitely an advantage. The reason being is that the recruiters follow up quickly and make sure that the hiring managers are moving quickly and fully considering you as a candidate. Not using a recruiter you will most likely get lost in the shuffle and/or you will have to wait weeks instead of a day or two for feedback. The recruiters sell you well - at least the good recruiters do.

For the life of me, I don't fully understand it as like you said the employer can save 15% or 20% by not using a recruiter. But for some reason that doesn't seem to matter.