I work for the software division of an international wholesale chain, based in Europe. For the checkout process, not only we have to translate everything in tens of languages, but we have to implement many legal requirements in price calculations, taxes, discounts, money laundry, invoicing and so on.
Even a single country like India is a headache because they have different legal requirements in each Indian state.
When you add countries from Europe which aren't in EU, countries from Europe which are in EU but have different legal requirements, countries from Asia, it can become really complex.
And we are just a handful of people and not payed a lot.
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.
The complexity of getting our platform to work with all of this is as big a consideration as market size is when we're looking to expand abroad.
If two start-ups tackle the same problem, the US startup can get 100x more money. Guess who will be the winner?
Do you believe Elon Musk could have launched Tesla and SpaceX in South Africa?