It's not that the US economy is better than others, it is just trusted to be the default place for people to invest their excess money which has resulted in most of the US stock market being overvalued compared to the rest of the world.
"U.S. stocks' outperformance on average over the past half-century or so has simply been due to increasing price multiples, not an improvement in business fundamentals. That is, U.S. companies did not generate more profit than international companies; their stocks just got more expensive. [...] Cheap stocks have greater expected returns and expensive stocks have lower expected returns."