Because people are still willing to pay those prices - whether because they don't know any better (i.e. the seller's basically taking advantage of ignorance, even if not willful), or because the buyer assigns that value to the object based on other characteristics (maybe trying to complete a collection, for example).

A secondary thing is that there's a minimum value for these things anyway - people have to put in time for packaging+shipping, the packaging+shipping itself has a cost, and below a certain base value for the product, the aforementioned aren't worth the bother.  So while a Block Erupter USB, when calculating $/Gh from say an Antminer S7 would be almost worthless, a more reasonable market price is between $5 and $10.  Lower than $5 and you'd be better off yanking the heatspreader off and try selling that separately as 'jewelry', for example