Maybe the momentum of Stoxx600 will last the next 4 years? Or maybe the S&P500 will come crashing down soon? Who knows.
The Shiller PE ratio is insanely high. At least the European market isn't completely overinvested in just 7 companies who are spending a lot of their money on the exact same thing, so it has that going for it.
Cumulative returns are around 100% for the american index, vs 60% for the EU one.