It's beyond that – culture. Being ambitious in US is rewarded and celebrated. Plus having the most powerful military in the world makes others want to invest more into the US due to the likelihood it will still be around.
1. Even if your idea seems crappy, investors will give you millions to bootstrap it. Uber, Twitter, Doordash, AirBnB, Coinbase, Instacart - all sounded funny before they became billion dollar behemoths.
2. Social norm of hire fast, fire fast - allow startups to quickly scale if they sense some opportunity and quickly ramp down in case that doesn't pan out. Again, different countries have different trade-offs around this. But the American tradeoff makes it much easier to build and grow a startup.