> Well, being the only country left with any manufacturing after a catastrophic world war is one major reason.

This is not the reason. Post WW II Europe was able to rebuild and for a long time they had some of the biggest companies and for a long time EU as a whole was a bigger economy than the US. It's only in the last 2 decades where the US is significantly outpacing the EU. And there are no signs of slow down.

WW II can not be the reason EU has few big tech companies. (relatively compared to the US, few exceptions always exist)