Here's what you're missing: I do a lot of online shopping, but packages getting stolen have cost me, personally, $0. Your proposal would cost me > $0. Why should I participate?
I understand that this might be how you feel since the seller probably has refunded missing packages. Now please consider that the added cost to the seller has already been factored in on all your purchases. Therefore, if there were no stolen packages, your purchases would have been cheaper. Does that change how you feel about it?
Loss from package theft from doorsteps is still less than retail shrinkage. It's difficult to get exact numbers, but even in densely populated areas it's unlikely to reach the retail shrinkage of 1.44%.
If package theft decreased from, say...1% to 0.1%, that would not substantially impact price on the micro scale.
That's pretty much a 0.9% reduction on overall costs to the seller. Most credit cards offer 1% cashback and people love it, so I will dare to say that a 1% price reduction is very interesting.
There's a few benefits that a solution could provide: (1) protection from weather like rain or strong wind; (2) protection from nosey neighbors finding out about sensitive deliveries; (3) long-term storage for deliveries when you're on vacation; and (4) protection from theft.