If you're talking directly to someone about buying their house and you go, "your house is a piece of shit, everything is broken, knock $XX,000 off or I walk", they're going to tell you to go fuck yourself. Once you filter it through two agents they hear "well, the buyer is concerned about X, which is very reasonable, but Y is just being picky, we should come back to them with an offer for $ZZ0,000 and I think that's very fair." Sure, they're not happy about selling their house for less, and maybe no agreement can be reached, but the agents take a lot of the emotion and confrontation out of haggling so that you don't end up hating the other party before you can reach an agreement.
So it goes with all negotiations.