Most of the fundamentals of supersonic aerodynamic design, the jet engine, the radar, the first rocket engines, the first RAM (for German subs in WWII BTW) come from Europe as from Europe comes the mathematical foundations to solve the design for stealth. Even the chief designer and boss at Skunk Works during their stealth breakthroughs was a European. The yanks are quite good, they stole an awful lot of intellectual property in Germany after 1945 (like the other victors), they took scientists across the ocean, and learnt a lot. Threw a lot of money at the problems and moved very quickly forward. They were better in those days, I must admit...if I look at the pace of the JSF programme.
So it is not they can´t but rather they did not want and they were blocked, anyhow.