As others have pointed out in this thread, the "porch pirate" problem is just kind of not a problem for most people/places. I've never heard of anyone I know having a package stolen off their porch. This is even living in relatively high-density (for the US) apartment complexes. Leaving stuff on the porch is basically good enough, so nobody is in a hurry to "fix" anything about it, except maybe Amazon themselves because even something that happens 0.1% of the time is a big enough problem when you operate at their scale.
I suspect that some of the the same reasons that lockers aren't economically feasible in most of the US is the same reason that the theft isn't a problem: low density. If you're a porch pirate, you need to expect that the value of your stolen goods covers at least your gas and time driving around stealing stuff, plus some risk premium for doing the crime. If the average value of a package is below this amount, the crime doesn't pay. There will still be instances where people haven't done this math, or crimes of opportunity, or just dense stretches where it does make sense depending on the price of gas, but it isn't a nationwide problem.
I suspect that some of the the same reasons that lockers aren't economically feasible in most of the US is the same reason that the theft isn't a problem: low density. If you're a porch pirate, you need to expect that the value of your stolen goods covers at least your gas and time driving around stealing stuff, plus some risk premium for doing the crime. If the average value of a package is below this amount, the crime doesn't pay. There will still be instances where people haven't done this math, or crimes of opportunity, or just dense stretches where it does make sense depending on the price of gas, but it isn't a nationwide problem.