Our guess is that:
1) Seller convenience, you don't necessarily need to pack up a box since it's being hand-delivered. Whereas with UPS/FedEx, you'll have to put something in a box so that it doesn't get destroyed during shipping.
2) Since it's getting picked up by an individual (sometimes on bikes), it's nice to know if they're going to be handling something very fragile or sturdy.
Just our assumptions - Uber will of course have the real answer.
