The thing you're missing is that government debt has an automatic expiry date to it, for example 10 years. So the question is, if I lend the US government my money for 10 years am I more or less likely to get it all back than if I lent it to a third world nation?

With the US gov whether the republicans or the democrats are in power I can reasonably assume I'll get my money back, can I say the same about the various political parties in a third world country were they to be elected?

The AAA/AAB style ratings are simply estimates of that reality.