Think about how bikes are dated-- most often, the advice is to check the components for date codes. So what happens when somebody buys a vintage bike, and strips it down to the frame for conversion to fixed gear? The date codes for that bike are lost forever. Then add in all the conversions that remove all trace of the bike's origin, and you have an unknown bike of unknown vintage. Nobody will ever be able to sort these bikes out. When the fixed gear crowd moves on to some other hobby, the vast majority of those bikes will be abandoned as worthless scrap.