That's not how averages work. If 9 people have an IQ of 100 and 1 has an IQ of 10, the average is 91. 9/10 people are still above that.
Also, human IQ is distributed in such a way that the mean is approximately also the median, which, by definition of IQ, is 100.
The reality is if you measured absolute intelligence that it would not be a standard distribution (there are more people at the bottom than expected), but it would be close. The reason why is that intelligence is the result of tens of thousands of independent factors (mostly genes).