Because of the historical high cyclically-adjusted price to earnings ratio (CAPE) of the US market [1]. I agree that historically low interest rates counter part of this argument, but overall, I still think there will be a reversion to the mean. Nobody knows if it'll be a sharp correction, or if prices will stagnate until earnings catch up.

Maybe I shouldn't have said risky. You're right that if retirement is a while away, you can continue not timing the market and you'll be okay. However, if you want to be more aggressive, it might be smart to sell some of the gains you've made in the US market thus far and allocate more to emerging market ETFs with more upside.

Using the CAPE ratio to project estimated returns has shown to be fairly accurate from 1995 to now [2]. There's no guarantee it'll continue to be true, but I think there's more upside to international equities for the next decade right now.

[1] https://www.starcapital.de/en/research/stock-market-valuatio...

[2] https://www.advisorperspectives.com/articles/2020/07/20/the-...