If you add the Eurorack format you get a couple of manufacturers based in the US: For example 1010music (Los Angeles, California), 4ms Company (Portland, Oregon), Acid Rain Technology (Seattle, Washington), Buchla (Minneapolis, Minnesota), Five12 (Albuquerque, New Mexico), Haken Audio (Champaign, Illinois), Industrial Music Electronics (Portland, Oregon), Make Noise (Asheville, North Carolina), Noise Engineering (Los Angeles, California) and Pittsburgh Modular (Pittsburgh, Pennsylvania). And then you have companies that are involved in audio technology and recording like Apogee Electronics (Santa Monica, California) or PreSonus Audio Electronics (Los Angeles, California). And then there's of course Spectrasonics (Burbank, California) in the purely digital realm.
However, no hardware synth manufacturer uses parts exclusively manufactured in one country, though, as not everything is manufactured everywhere. No matter where the synths or modules are assembled, the complete manufacturing process is typically a global affair that involves suppliers from all over the globe.