The trouble with comparisons like this is the lack of scale and perspective. Australia and Taiwan are roughly half the size of California. The United States has a far larger size, and a far greater regional diversity.

If you draw comparisons across similar population sizes and similar geographic regions, for example comparing the USA to all of Europe, the numbers look a lot more similar:

Europe median: $26k Europe mean: $174k

USA media: $82k USA mean: $505k

Both regions have roughly a factor of 6 difference between median and mean wealth.