Not as much lazy as following incentives. They would rather invest their time (and network) on higher paying jobs that are likely to close.
Scenario 1: A larger company can pay 20% * 120K on a hire, that's $24,000. Let's say that there's a 1/3 chance that the headhunter can fill it. Expected value is $8K. Let's say it takes 100 hours of work, then it's $80/hour.
Scenario 2: A smaller company won't pay as much, will take more work finding people interested, and they may not agree to take the job. Then it could be 20% * 100K = 20K. Perhaps a 1/4 chance of closing the deal = 5K expected value. 200 hours to find someone = $25/hour.
The recruiter would rather do the first rather than the second.
Scenario 1: A larger company can pay 20% * 120K on a hire, that's $24,000. Let's say that there's a 1/3 chance that the headhunter can fill it. Expected value is $8K. Let's say it takes 100 hours of work, then it's $80/hour.
Scenario 2: A smaller company won't pay as much, will take more work finding people interested, and they may not agree to take the job. Then it could be 20% * 100K = 20K. Perhaps a 1/4 chance of closing the deal = 5K expected value. 200 hours to find someone = $25/hour.
The recruiter would rather do the first rather than the second.