Because they either source, warehouse, or on-demand fabricate overseas. This was kind of shopify's original model, remember when suddenly everybody knew what drop shipping was? Anybody could make a virtual store and you didn't have to buy the product until the moment you made a sale and the wholesaler will "drop-ship" it directly to the customer, meaning the seller isn't part of fulfillment. Welcome to just one part of the economics that made that cheap enough to be viable, and it was sold to their customers as a low risk way to get started. But if you look carefully the responsibilities are structured in such a way that liability is on the you the customer and very briefly the fulfiller, who immediately hands liability over to the postal services, at a significant discount, until finally it reaches you. This is capitalism 101, risk is mitigated by placing a structural intermediary. Think about the gig economy, if all these people are contractors then they inherit a significant burden in the form of those additional liabilities. That's why they fight tooth and mail to not have them classified as employees