Population aging and growth uncertainty is a big guess on my part.
https://www.imf.org/en/News/Articles/2020/02/10/na021020-jap...
The US is highly dependant on massive amount of immigration to maintain demand. Couple that with a few other things, such as the petrodollar, and you have the US behaving in an odd manner that I don't believe will last forever.
Japanese companies are notorious for having poor return on equity for decades.
Government deficit spending of course helping the economy. But we eventually have to pay taxes to cover that.
Citizens debt fueled spending on homes and goods. Corporate debt similarly. Student loans of course. Individuals and companies can only tolerate so much debt.
Our oil dependence never accounted for the cost of global warming and pollution, but we’re about to pay for that soon. This applies to many materials we consume, oil being the most prominent.
Corporations have systematically slowed pay growth while increasing prices, eventually consumers will be unable to afford enough goods to keep the machine spinning at full (growing 7%) capacity.
Our shrinking population from historic highs means each working person will need to contribute a bigger share to reach that 7%, while having more people to support.