The dealer is in business to profit. Why would he offer anything close to retail value?
Privately selling used cars is not only inconvenient, it can be costly. Dealers pay for staff and facilities to clean and repair trades, advertising and prime real estate to attract buyers, regulatory fees and taxes, additional staff for sales and accounting, utilities, etc. They are also taking a big risk on a trade since those trading a car in can be just as dishonest as the worst car dealer out there. Any concealed/undisclosed damage or issue is their problem once they accept the vehicle. In some states the dealer has to guarantee the car will pass inspection even when sold as-is. He has to deal with all the lookie-Lou's that just want to joyride and waste his time. Even if he just turns around and sends the car to auction there are transportation, clerical and auction costs involved. On top of all that, the dealer owner would actually like to make some money for himself.
Trading a car eliminates a lot of risks and allows the new car buyer to perfectly time the sale of the old car. That's worth money to a lot of people. Same goes for things like scrap metal. If you call a scrapper to take a junk car away, do you expect him to pay you close to the actual scrap value of the car, or do you think it's reasonable for him to want to cover his costs and still make some money for his time and effort?
It's almost as puzzling to me when people think that what the dealer paid for the trade has any bearing on the selling price. The value of the car is the value of the car. When he takes a trade that ends up having major issues that need to be fixed, would you offer to pay him more for it since he had to put more money into it? Let's say a Subaru dealer took a VW TDI on trade the day before dieselgate hit the news. The market value of the car dropped significantly but he still has the same money in it. Can he still get that or does the actual market determine what he can sell it for?
I wouldn't call a low offer on a trade unfair or dishonest. The dealer is telling you what the car is worth to him. Is it dishonest when a buyer low balls the dealer on the new car?