Commission is typically 6%, which is split 4 ways to the buying agent, their agency, the selling agent and their agency. So your agent personally takes home 1.5% of the selling price -- on a $300,000 sale, they are making $4,500.
The difference between selling your house for $300,000 and $310,000 after commission is $9,400 in your pocket -- and a paltry $150 for the agent. Are they going to take the risk of leaving it on the market for an extra week (and spending more time showing it, etc) hoping to make an extra $150? Of course not. (Spoiler alert for the book: unless it's their own home, in which case they leave it on the market for an extra 10 days and sell for an extra 3% on average).
Another incentive problem exists on the buy-side: your agent gets paid more money the more you pay for the house. It's literally working against their interest to negotiate a lower price, other than when it's above your absolute maximum.