Because outside of the quick CDC arc, they've just ignored the outside world and focused on our small merry band of zombie slayers. While it's more intimate this way, I wouldn't mind a peek at the larger issues occasionally, even if it's just showing us that the world is borked and not coming back any time soon. At least show us some more failed attempts, or to use your example, that the US tried to hang on, but that carrier eventually failed too...
This is true and I as well would love to look at the larger issues. If AMC isn't already planning either after TWD's enormous success:
1. A spin off perhaps mini-series about what we are discussing
2. A full fledged movie about it.
They are idiots.