Because of the Bretton Woods system and the subsequent Nixon shock. The combination of these meant the USA can print much more money than anyone else, give it to whoever it likes (it always ends up at shareholders), and the resulting inflation is the whole world's problem. Because the USA prints so much money that ends up at shareholders and some at bondholders, the rest of the world buys US shares instead of the rest of the world's shares. The US ownership class basically ends up with so much money and not enough things to do with it, and some of it gets put in these risky technology bets. Other side effects include low wages, since US owners don't have to pay workers to make good products as much as they do in other parts of the world, preferring to just siphon off the endless free money stream, and high non-financial asset prices (e.g. houses), since there are so many wealthy owners to sell to instead of workers.
And the Bretton Woods system is basically a side effect of the fact that Europe had World War 2 and the USA didn't.
