Generally speaking, we interact with people similar to ourselves. People with whom we share similar interests, experiences, heritages, etc. One reason for this is that there are very few activities that span the Big 3 social divides: economic status, race, and age.
Thanks to Southwest and other bargain airlines, flying is one of the few – and possibly only – occasions in which we are actually exposed to the general population in America. All other forms of public transportation exclude the rich and almost any other activity (entertainment, food, work, etc.) pools people of similar interests and/or backgrounds.
It’s not that there is a disproportionate number of weird people at airports (as I had previously thought), but that there are lots of weird people everywhere. We just don’t run into them very often in every day life because they don’t work/shop/eat/live where we do.