That feels a lot more like correlation than causation.
What you could argue is that because these companies are so successful, they can now offer disproportionately large compensation packages including stock options which draw talent from the rest of the world to keep them large. This in turn fuels the startup economy as investors dream of having a big share of the next big thing.
I don't think it makes sense as a justification for why these companies grew so big and attracted so much money, causing a gap in startup environments. It would at most explain why they stay so big.
American economic success is not due to old companies staying big. It's due to the immense rate that some startups tend to grow.
From the 100 most valuable companies on Earth, there are maybe 20-30 US (rough guess) tech companies that have been started in the the past 40 years, and several of them in the past 20.
And approximately 0 European ones.
And while this isn't PROOF of causation, it surely increases the posterior probability of causation, regarles of what your prior was before adding this evidence.
What you could argue is that because these companies are so successful, they can now offer disproportionately large compensation packages including stock options which draw talent from the rest of the world to keep them large. This in turn fuels the startup economy as investors dream of having a big share of the next big thing.
I don't think it makes sense as a justification for why these companies grew so big and attracted so much money, causing a gap in startup environments. It would at most explain why they stay so big.