Earlier quoted context omitted.
The problem is that capital is mobile, and that mobility is arguably increasing due to digitisation. In other words, if a country raises tax rates on returns to capital, capital simply relocates somewhere else. I've suggested a solution above (or it might be below by now), but it's complex...
USA already has worldwide tax jurisdiction. The more difficult problem with capital taxation is how to accurately value an asset.
The Large global companies spend millions on ultra complex business structures to avoid US (and EU) taxes.
See GE and Apple as the most publicized examples but they all do it