It probably doesn't pay off to make one shirt/underwear/cup etc. at the time.
You take lets say 20 orders and then make a order from a factory, getting some kind of discount etc.
With that "exocit" stuff (cups, underwear) it would take to long to get whatever number of orders is needed to keep it profitable,
and people would wait for example 6 months for a cup (imagine the moaning threads then
).
Dunno if it's like that, but seems logical to me.
If you want something exotic like that, nothing stops you to design something simple and order over net shops whatever you want.