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.
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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.