That graphic shows ~44k HNWI emigrating, and ~20k HNWI immigrating. I skimmed the underlying text and didn't see an explanation, so are we to assume the other ~24k HNWI immigrated to the rest of the world's countries in relatively low numbers per country?
I suppose another answer might be that it's easier to track HNWI leaving countries than it is to track them entering other countries?
While funny, California as a whole has (according to Wiki) 8.5% of million dollar net worth families. There are almost 10million of them in the US. That's on the order 1 in 10 people belonging to a family in that bracket in the US and thus a whole bunch of people. So that's not really just some tech bro's in the valley...
But I think when you extend the sample to the spectrum of all humanity (as the infographic does), $1 million USD becomes quite clearly a fitting benchmark of wealth.
I suppose another answer might be that it's easier to track HNWI leaving countries than it is to track them entering other countries?