Because the bigger companies were established long ago, when the US had the much bigger market. Nowadays that's not true but the US startup VC is a lot stronge

Because the bigger companies were established long ago, when the US had the much bigger market. Nowadays that's not true but the US startup VC is a lot stronger in the US for a bunch of reasons, so new startups tend to be established there much more often (with the financial tech field being the exception I believe). If you're making a new company you're going to make it where you live just out of sheer convenience.