Yeah I feel like that's a highly opinionated claim.
Some of my favorite bands are
From Sweden - In Flames, The Hellacopters, Disfear, Amon Amarth, Cult of Luna
From USA - The Gaslight Anthem, Bad Religion, Fleet Foxes, Bon Iver, Robert Francis, Greg Laswell, Converge
From England - Mumford and Sons
From Australia - Midnight Oil
So yeah, most of my favorites are from America. That doesn't mean the best music is from there.