Recruiters are paid a percent. The most important thing for them is a sale. Not the amount of sale. So for example would they spend extra time getting you 5k more a year or 10k more? No. If they know you'll accept a lower number their goal is to keep working with the employer.
What is better? 20k payout? Or 20.5k payout? Practically no difference. To you an extra 10 or 20 grand a year is huge. To them it's nothing. They care more about the deal happening than optimizing it.
After all at the end of the day:
1) The employer wants to pay the worker the minimal.
2) The worker wants to get paid the most.
And since recruitmen agencies are employed by the employer this is obviously gonna happen.
Perhaps the true issue is that people don't understand that negotiations are a polite form of "cut-throating"?