That used to be the case in the past. However with the increased government expenditure on basic science in the wake of the cold war, more of the fundamental science did happen in the US (foundations of CS, foundations of Crypto, lot of AI & ML, string theory, lot of work in Math, etc.). Now with the decreasing focus on the funding of science, fundamental research (especially in high energy physics but in other areas too) is again shifting to other places like Canada, Europe, Israel, China and Australia.