Probably because of returns/scams. It's too easy for buyers to make a "not as described" claim or other reason for return and then the seller is expected to foot the shipping bill for the return (as well as the original shipping AND hope they don't get an empty box returned).

Side note: I actually did return an empty box to a seller once because they never sent me the item and due to some eBay stupidity (order was bundled with another item I did receive), I had to do a return to get a refund instead of an "item not received" case.