It's exactly the other way around. The USA is not a developing economy, we do not suffer from a lack of capital and abundance of investment opportunity, we suffer from an abundance of capital and lack of investment opportunity. The average American billionaire has no idea what to do with a marginal dollar, so he just bids up assets with it and maybe punts on spaceships or something. The average American, in stark contrast, spends the dollar satisfying very real as-yet-unsatisfied consumptive wants and needs, at which point the dollar gets spent and taxed and spent and taxed again and again and again. That's understatement: the velocity factors are 0.7x and 5x, last I recall.

You can read the balance of explanations off interest rates, you can read it off of valuation metrics, you can read it off of judgement calls about the quality of the marginal investment opportunity. You can't read it off the anus of a billionaire or the turd of self-serving think tank propaganda it pinched out, though, and that's where you are clearly looking for it.