I’m not sure US academia is mediocre. It’s more like… normal?
But America being what it is, it attracts those with most potential creating and sustaining a network effect.
But there’s nothing intrinsically good or bad of the US, and it’s quite easy to mess up the equilibrium and go back to the mediocrity you mentioned
It’s a numbers game. Taking the best from the world talent pool is going to provide better results than from the much smaller American talent pool. Unless your country has more than a billion people, you need to look at world talent.