When comparing historical gains (I used the "adjusted close" data on yahoo) of major indices in different countries it is clear that the US outperformed most of these by quite a bit (see for example also https://www.visualcapitalist.com/worlds-major-stock-markets-same-scale-1990-2019/). From US based sources you often read that the expected average yearly return on investments is around 10%. This figure is based on historical data of for example the Dow Jones Index. However, other major indices had much lower performances (on the extreme end: the Nikkei 225 had basically no gain in the last 30 years).
Is this just a matter of the US having been the (unpredictable) "winner" over the last 100 years (or decade or most time intervals in between)? Just like having picked Apple around 2005 would have been very lucky. If so any prediction on the new average return on investments should clearly not be based just on this "winner" from previous years right? Would it not be more accurate to base the estimate for the average expected return on more (all) markets?
Or does the US predictably outperform other markets for some reason? That would beg the question why all markets are not emptying out and pouring their money into american stocks.
Is there some tax, regulatory, currency or inflation related reason that a consistent predictable gap in the average gain between US markets and other markets would not close?