Only Uber (and other ride-haling apps I suppose) forces a price, all other marketplaces the price is set by the seller - AirBnB, Etsy, fiverr. In Amazon Turk the seller and buyer roles are reversed and Turkers only accept work that pay enough for them.
I would say it's more a product of the industry they are in - transportation is usually regulated to have a fixed price almost everywhere - then the nature of marketplaces in the gig-economy as a whole.