It's easier to grow scale in the US because of the homogeneity of the culture and language, and the integration of the economy.
Competitors in Europe or elsewhere are relatively small potatoes in the medium to long term, because the lack of homogeneity means they won't get scale quickly enough. US companies can usually either outcompete them with better technology (cost amortized by scale), or buy them out.
Competitors in Europe or elsewhere are relatively small potatoes in the medium to long term, because the lack of homogeneity means they won't get scale quickly enough. US companies can usually either outcompete them with better technology (cost amortized by scale), or buy them out.