A lot of natural processes produce results that are normally distributed. So given a large sample, a continuous value range, no bounds (or far away bounds) and

A lot of natural processes produce results that are normally distributed. So given a large sample, a continuous value range, no bounds (or far away bounds) and a lack of obvious skew the intuition "half the population is below average" has a good chance to be (approximately) true. Of course IQ is a made up thing that we just defined to behave that way.