PayPal favors tracking numbers. It's an impartial judgement system. So if some jerk seller wants to send a brick in a box instead of the widget ordered they win. Anything other than a tracking number would involve human interaction which is time consuming, expensive and wrought with problems.
The tracking number issue works both ways. If a buyer makes a SNAD claim they have to send the item back to the seller WITH tracking to prove they sent it back. PayPal needs online, irrefutable proof that a package has been delivered. If tracking shows it has, then it has.
Look at all the stupid answers you get from folks answering questions at eBay's Live Help. Do you really want one of them to sit in judgement on a claim? I don't.
