Because (provided ur dude is telling the truth) the recipient did not receive the package. It's not so cut and dry where the usps is gonna say "it says delivered, f u". There's way more to it. They will investigate. I've dealt w usps on lost,wrongfully delivered, u name it. And they won't just give u the insurance $, they'll refute your(or ur guys) claim that it wasn't received all the way down to them contacting the carrier and the carrier saying "I handed to this dude". Especially if u had it insured.
The truth will surface.
Like the others said above, I think ur dude is FOS.... But if u need some assistance in coming to that conclusion then usps will assist.
Just as a sidebar, I'd make one last contact w the recipient and tell him that ur hitting up usps to do an investigation. This gives him that chance to say "it's all good, the neighbor accidentally got it" wink wink