"The USA" doesn't mean a lot now that private companies have sprung up. You do have a point, however: NASA is damn good at payloads. Their stuff usually works really well once it's up but, well, they kinda suck at launch vehicles. The most likely reason is pork. It's hard to spread out manufacturing of a space probe over fifty states but a new launch vehicle? Easily. And once parts manufacturers exist they must never go away, hence the SLS, which most likely will be unneccessarily expensive while performing worse then a vehicle that wasn't designed to generate revenue for every damn state in the union.
Honestly, NASA would probably do better if they sold their launch assets to SpaceX and focused entirely on payloads and missions. Whether they'll end up launching with rockets from Russia, Japan, SpaceX or even Copenhagen Suborbital is no matter as long as the rockets are reliable. The existing rockets have a track record and it's often a good one. NASA's new launcher doesn't.