Sometimes we'll negotiate just because the candidate wants to maximize the offer, but negotiation isn't usually as intricate or time-consuming as it's portrayed.
It does benefit me to close a deal quickly, but the easiest way to close quickly isn't by convincing candidates to take low offers - the best way to make a deal happen quickly is to convince the client to pay my candidate at or above market rate.
If candidate tells me $n gets their acceptance, the easiest way for me to close that deal quickly is to get $n+5 from my client - not to convince candidate to accept $n-5. And $n+5 also nets me more than $n-5.
The risk isn't the time spent in negotiations over a small difference to the recruiter's bottom line - the time is minimal. The risk is losing the deal entirely (and getting $0).