> but I think share buybacks are another major reason for the disproportionate rise of US markets. US corporations have bought more of their own shares than anyone else in recent years
Share buybacks are just a more efficient way of returning profits back to investors than dividends [0].
> Why can't corporations find anything better to do with that money? Why is capital spending relatively muted while productivity growth has been subdued for years
Most companies are demand limited which limits their investment opportunities. Also, you want to move capital where it can get the highest return. If a companies best investment opportunity gives a return of a measly 2% a year when the market is doing 7%, than you should not do it and instead return that money to investors so they can divert their investments to companies with higher returns.
[0]: https://en.wikipedia.org/wiki/Share_repurchase#Tax-efficient...