Because Silicon Valley, or the current best place for stuff like this, in the US. Can have a wide range of engineers, with just the right sorts of experience. With further ones, poachable possibly available from existing companies near and far, in the US, and ones who are happy to come and work in the US.
Currently the best place, maybe. In 10 years time when the effects of the gutting of US science are felt, not so much.
And, of course, add in all the incarcerations of tourists and (illegal) deportations of people with valid visas, and who would want to go there.
I worked happily for (decent) US companies for 20 years. Nowadays I would look elsewhere.
I guess the answer is, that things change over time. I still think 10 or 20 years, is a long time in the future, and things could go either way. I.e. The US could get better and better again, or it could carry on, on its (apparent?) downwards spiral.
E.g. In the era, around 1929, The Great Depression. Things could look rather bad for the US. With some thinking (perhaps), that it would be in an even worse state in 10 or 20 years time.
I think I've read (IIRC), that actually things were reasonable (post 1929), in the US (not in a bad way, but other countries round the world, were more powerful (military wise, at least in some respects, such as navies), maybe richer, or as rich, and more industrialized etc, in some respects). I'm NOT exactly clear on the details, so take it VERY roughly.
But it was the advent of World War 2, and the massive destruction, harm and removal from the world stages, of many countries round the world. That helped propel, the US, into the great (opinions can vary), position the US found itself, in the years between the end of world war 2, and recent times.
E.g. The First moon Landing, is rumored?, to technically be as a result of Germany's development of ballistic missiles (e.g. the V2), rocket technology and the mass removal/transfer of many prominent Physicists and rocket scientists of the time (from Germany) to the US. That helped them be able to develop it (the rocket), and be the first to (manned flight) land on the Moon.
Along with other technologies (such as Nuclear bombs, via the UK and some of those Physicists + many Americans), ballistic missiles, the start of modern submarine technologies, and maybe other stuff. But to avoid massive walls of text, I will stop now.
Edit: Attempted to improve historical accuracy.