[Chrisoaks]

Because the price paid is somewhat less than the value derived. You might have been willing to pay him $110 and he might have been willing to walk away with $90. But regardless of whether you settle on a rate of $90 or $110 or split the difference at $100, a $20 positive sum is created.

You mention competitive markets, where surplus is reduced or almost eliminated. But most markets do not reach such a late stage level of optimization.