Is there a good reason to invest worldwide?

This article shows that US accounts for 52% of total world stocks.
With Japan at 7%, UK at 7% and EU at 7% trailing behind.
Investing in any non-US market is of higher risk, especially monetary risk, but also growth of developed markets and market manipulation risks come to mind.
With highly differentiated monetary politics, it makes sense to invest in currency hedged developed stocks (HEDJ, SCJ) or premium growth countries (ASHR, INDL).
Both of these alternatives are higher risk than SPY, and should be considered mainly for diversification purposes.