And a place like Amazon that sends out lots of orders will get a good idea of where the risks are high and where they are low. They know that of the hundreds of packages they have shipped us none have gone missing, it would be a waste of money to insure things they are sending us.
They even screwed up and shipped a cancelled order, I was on the other side of the world when I saw the e-mail and by the time they were satisfied I really was me their attempt to intercept the package failed. $500 worth of stuff sat on our front porch for 10 days and was still there when we got back. (We live in a low-crime area and our front porch makes it basically impossible to see a package from the street unless it's large. A porch pirate would have to come to within 10' of the door to see most things, they'll go elsewhere where they don't have to be so obvious.)
I don't know how this works in general but I assume that there are some heuristics as to whether shippers require delivery signatures or not. Because of where I live and how isolated my house is the only real dangers are misdelivery and damage.
One of the early-on Prime benefits for me was fairly deterministic delivery. At a time when I ordered lot of physical books, CDs, and DVDs, the minimum amount for free shipping wasn't a big deal. But random order arrivals when I was traveling much of the time were.
They even screwed up and shipped a cancelled order, I was on the other side of the world when I saw the e-mail and by the time they were satisfied I really was me their attempt to intercept the package failed. $500 worth of stuff sat on our front porch for 10 days and was still there when we got back. (We live in a low-crime area and our front porch makes it basically impossible to see a package from the street unless it's large. A porch pirate would have to come to within 10' of the door to see most things, they'll go elsewhere where they don't have to be so obvious.)