I suspect this actually starts to change at the upper end of the market, but only if you have proportional commissions.
There aren't that many $10,000,000 houses. You can't just dump a $10,000,000 house ASAP and list another $10,000,000 house next week. And at $10,000,000, every percentage point in sale price is worth a marginal $3000 to the sellers' agent. If someone comes in 10% below listing on a $10MM house, the difference alone is equivalent to the sellers' agent commission on an entire $1,000,000 house.
There aren't that many $10,000,000 houses. You can't just dump a $10,000,000 house ASAP and list another $10,000,000 house next week. And at $10,000,000, every percentage point in sale price is worth a marginal $3000 to the sellers' agent. If someone comes in 10% below listing on a $10MM house, the difference alone is equivalent to the sellers' agent commission on an entire $1,000,000 house.