John William
Well-known Exeweb poster
But often it's more than just simple "supply and demand", though. Its about perceptions not supported by objective information. And maybe the pan-human need to identify with a tribe?For the same reason footballers get paid millions of pounds I would imagine. Supply and demand.
To focus on football, objectively goalkeepers are the most important players on any team, as their errors are more important, but they are not glamorous so command lower fees and lower wages than strikers. Similarly their shirts are rarer (only 2 out of 20+ in a squad) but they are not so valuable.
Another good example is "celebrity" autographs. Mark Hamill (Luke Skywalker) recently said he is stopping signing stuff as there are so many forgeries around, or if he does will personalise them so they are less tradable and more verifiable.