The problem is that we don't know what the thresholds should be in advance. With manufactured goods, it's easy to compare. Salesperson A sold 10 Toyota Prius Base models for $18,000 and Salesperson B sold 10 Toyota Prius Base models for $19,000. Houses can't be directly compared.

If the seller is in charge of the thresholds, they can just set them unreasonably high. I can look at my house which would be reasonably valued at $500,000 and say that Threshold1 should be $750,000 and Threshold2 should be $800,000. The agent will never be able to meet those thresholds.

If the thresholds are with respect to the listing price, it's easy to just list high and then accept a lower offer. If Threshold1 is 5% above listing and Threshold2 is 10% above listing, people will just list their houses higher.

With cars, I believe most places do laddered commissions. However, it's easy to know the value of a bunch of manufactured goods that are all comparable. With housing, you can't go by square footage. Some places are nicer than others. Even within a neighborhood, some areas are more desirable and even different sides of the street might get better light.

The incentives are definitely misaligned, but I think it's hard to align them given that they're selling a good where we don't actually know what the value is and where each sale is pretty unique.