Employers already do - but they pay places like linkedin, craigslist, monster.com and every other job board out there instead of paying you. Listing a job can easily cost $400.
* the agency slice of the pie if an agent is involved (often at least a five digit number)
* the people costs on the employer side of recruiting (writing job specs, managing adverts, weeding resumes, phone interviews, actual interviews, etc.) can easily take weeks worth of time from multiple people.
Recruiting is expensive, and hard ;-/