Because that's their choice. Did you read the part about never losing a package? WTF would I pay for it? If something got lost, I'd make it right, but I'm not giving the shipper anymore money than I have to. The odds are in my favor.
If the buyer feels more comfortable having the package insured, they can pay for it.
If the package is lost due to no fault of the seller, your premise seems to dictate the it's solely a loss on the seller to remedy, period. If you wish to cover it when you are the seller, so be it. Do you expect the same from others?
Just killing time. No big deal.
