I don't believe anybody could actually argue "all US beers are better."
But, the US has a great variety of breweries making all kinds of beer.
Allagash and Ommegang focus on Belgian Style ales, and they make some damn good brews.
Rogue, Dogfish Head, Three Floyds etc are all craft brewers who make a whole variety of beers. Many of them are fantastic!
Then there are smaller brewhouses that only distribute in their home state, such as New Glarus, who make wonderfull beers.
Then there's brewers like Goose Island, Stone, Anchor, etc...
Any and every variety of beer you've ever heard of, as well as pretty much all the ones you've never heard of, come from a multitude of brewers in the USA.
So, while stuff like Miller and Budweiser tastes like watery rice with an occasional hop, and they may dominate the mainstream market, they are not in any way shape or form the total sum of American brewing.
PS, Cask Ales are even starting to gain traction over here, it really is a beer paradise!