With the popularity and overall trendiness of SS/FG bikes, for a lot of people, their SS/FG is their first "real" bike as an adult. They don't want to spend a lot of money to get into a hobby they're not sure they'll like or stick with, so they go online to order bikes and parts. Not sure about sizing, or they read the wrong information on some website somewhere, or they measure incorrectly, or whatever, they end up ordering the wrong size bike. That or they ride the first/cheapest vintage bike they find. It's costly to return online orders and/or the rider doesn't really know any better since it's his or her first bike, so they ride it as is.