This of course is a generalization but it is the root of the problem.
I'd even go so far as to say it's decidedly optimized for recouping the investment in employees by offering a small percentage of the total expected value up front for those that can provide the proper candidates. However, I'd really like to see the whole process upended and brought more in line with modern capabilities.
I got all my best jobs through recruiters. Actually I even like dealing with them in general, they provide with tons of valuable info which you wouldn't know otherwise. As an added bonus they often amuse me when talking in a top-down way like 'get really prepared', ' this is a really good position for you', etc.
EDIT: I should have said that recruiters start talking to you nicely only after a certain point in your career. Till then, I guess, it can be a terrible experience and is better avoided by the means described in this thread
They are just tops if you want to get into a company that doesn't know how to hire good people. My experience has been that if a company is going through a 3rd party recruiter, their standards are ridiculously low (and thus, I look like some kind of minor god) which is great if you are desperate for the cash, and eh, being the big fish can be fun some other times, too... but it's not how I want to spend my life, you know?
Working for companies like that gets old pretty fast, especially when they make you do interviews... you end up interviewing a bunch of people who aren't worth $15/hr for $100K/year sysadmin positions... It can be pretty frustrating.
I think the thing is that when given a choice, most people avoid recruiters. If nothing else, that's a nice big chunk of change that could have either been a bonus or salary for the employee, or some extra profit for the company. I hear 1/3rd of a year's salary is not unusual... which is pretty huge, really.
so when you go through recruiters, you are competing with people who don't really feel they have better options.