Not everyone is driven by the bottom line. Many buyers and sellers have an ultimate purpose that may or may not include the notion that NO money should be left on the table.
Many people are generous by nature. They may very well know that they are paying too much or selling for too little, but the money isn’t all that important to them.
Acting in a buyer or seller’s best interest does NOT mean negotiating down to the last penny. Many a deal is lost over a few measly dollars.
Buyers want to get a property they want at a price they are willing to pay. Sellers want to move their property at a price they will accept. As long as the buyer has the ability to pay it is a fair market transaction.