Poor people are leaving to other parts of the USA slower than rich people are moving in from other parts of the world; California has net in-migration despite net domestic out-migration. This is bad in terms of domestic political power, of course, but it's directly a consequence of economic success.

It has lower net growth than many other parts of the country, but that is not just a matter of migration but also the lower natural growth rate that usually accompanies greater wealth. And it's not one of the places actually shrinking because people are actually fleeing, not being displaced by richer people—there are a number of states where that is true, but CA isn't one of them.