Its quite similar to vintage bikes..
When you arrive at the show, you feed of the reaction your vintage bike/shirt brings.. Is the 72 Ducati 750SS worth 75K? Is the ***** shirt really worth 12K?
Its all relative, we justify the value, because we need and can have..
Will the t shirt/Ducati, get ridden/worn a lot, and in the manner intended..
.. probably not, but a few people will enjoy seeing them..
So you see, they likely share a parralell obsession to vintage (high end) bike collectors.. not that hard to understand..
The clear winners, the folks that can find this stuff and bring to market at the high prices, that the market seems to be paying..
I would probably rather have a vintage shirt relaruve to my collecting tastes, than a diamond..