If half the population has an IQ below 100 why do they think they are above average?

I would assume the average IQ settles at 100 because roughly 50% of the population exists there. That is more likely from a biological standpoint than 50% of people existing on each side of the average. I think a majority feel that they are above average because we exist in a society that awards mediocrity and tries to eliminate the concept of "winners and losers", though we all know such a concept exists in reality.