Because it's lazily interesting races like "of <random> descent." You can try it with most models and get the same results. Try with prompts like "an ethnically white mechanic of african descent" or "a white german woman of hispanic descent" and you'll see that the non-white races win because images of white people aren't often labeled as such but images of other races are and have a strong association.

"Ethnically swiss woman in traditional ethnic dress of south asian descent" is, as expected, a bunch of south asian women.

The Gemini devs are idiots for thinking this would work, and it likely got shipped because for the random art prompts they were testing on it probably did look fine. But hot damn it's like folks just want to be mad about wokeness or whatever and got the torches lit before applying Hanlon's razor.