Language, culture, regulation - it's not just a matter of reach.
Like, imagine a successful-at-home French startup wanting to expand internationally. First, unless you're happy staying in the confines of the old French Empire, you're gonna need to translate everything, definitely to English, maybe to German or Spanish. Second, your app is built with French attitudes in mind. Chances are you're gonna have to change that to adapt to American, British, German tastes, and that's a lot of redesign. Third, for every country you expand to, you have to comply with local rules and regs. The EU has helped here, but definitely not solved the issue.
Whereas in the US, not only have you got 330 million wealth consumers under broadly the same legal/cultural regime, but they practically all speak English too! That gives US startups a much more solid base to expand from. Hell, in Europe the UK is by far the biggest tech hub on the continent, and Ireland punches far above it's weight. Don't discount the advantages of language or culture even in a globalised world.
Everything about US-based companies' success globally has to do with the fact that we've had a head-start on exporting our culture through media.
Everyone in the world has some idea in their head (regardless of how true) of what Americans are like. Can you say the same for the French? Germans? Portuguese?
> Everything about US-based companies' success globally has to do with the fact that we've had a head-start on exporting our culture through media.
A lot of it has to do with the fact that there is a close relationship between our companies, our government, and the application of our considerable intelligence and military resources, too.
I know someone that work in a startup that does exactly that. Opening a new country takes 3/6 months, as you need to open an office in the country and recruit people here. And that's now that they have a good process for it. Translation and localisation add processes and takes time. You need a new sales team for each country. Sometimes support too. Launching something in the USA would be equivalent to have France, Germany, Italy, Spain and the UK at the same time. That's a lot.