America has/had some very very good people of its own like Oppenheimer or Glenn Seaborg, Jack Northrop, Glenn L Martin, Clyde Cessna, James Smith McDonnell. Or in totally different fields Walt Disney, Norman Borlaug and Luther Burbank. Any of whom are world class.
But OTL the USA benefitted hugely from refugees from Hitler and Mussolini and Stalin. And co-operation post war from the European countries terrified of the USSR and British scientists whose domestic economy couldn't afford to fund their research ("the brain drain"). ITTL none of those refugees left Europe, the UK economy isn't in a parlous state and small countries like Belgium and Norway aren't giving away stuff for free (be it intelligence data or scientific research) because they have a national interest in giving the US an edge over the Soviets. ITTL if they can't exploit it themselves they can use it to buy kudos in multinational organisations, sell it at a commercial rate or use it to gain brownie points/exert diplomatic leverage with a larger neighbour. Which will be the UK, Italy, Germany or France, even Russia by now and not the US.
