The origins of Silicon Valley being a technological hub go back all the way to WWII and the economy of the early computer industry. So, I feel the USA being an economic and military superpower is a pretty easy explanation for why 'Silicon Valley' exists in the bay area today and not somewhere else. Obviously, there have been many years and many technological revolutions since those early days but there are pretty clear trails of people, and money leading all the way back.

As far as trying to answer why so many other countries consistently beat America in math and technology I think a small answer can be gleaned from your statement: "we were encouraged to pursue whatever we wanted to pursue." That is not all that common in many countries and cultures in the world. I think a lot of it has to do with economic or political desperation. I have worked with people who learned how to code because they didn't even have running water in their village in India and programming was a way out. Or more relevant to this story, they lived in a Russian town and got a PhD in engineering so they could come to America and be an engineer. These types of stories are very, very common in the Bay Area. Looking at the education systems is helpful, but I think you need a real driving force to get most people motivated to learn difficult things.