The vacuum and sudden reentry might be handled by the metal just fine, but remember, these things are loaded with explosives. (Kinetic weapons are effective, but also limited.) A thin metal shell that explodes miles up during reentry probably isn’t going to win too many wars. If it’s too solid, it might be unreliable for proper detonation when it arrives. As most explosives are purposely volatile, the casing would need to protect the contents, but not be a hindrance once the package arrives. Add in 40,000 G’s, and the other effects (lightning at the destination is one, IIRC), and you basically need a lunar capsule.
Of course, those aren’t cheap, which is what this really boils down to. They need something that can survive, is reliable, and cheap enough to actually justify it’s use. It’s nice to be able to sit at home and hit any location on the globe as needed, but if it costs too much, it’s pointless. Also, my understanding is that part of this program relates to the possibility of using these things for emergency relief, so no doubt that plays a (subtle) part in the statement, as well.
And let’s not forget, the main job of a military contractor isn’t necessarily success, it’s to keep the bucks rolling in. “Good enough” might work during wartime, but when you’re eyeing that Caribbean Island for retirement, it’s important to work towards “perfection”.