It’s about supply and demand. Luxury grocers provide a shopping “experience”, where low cost grocers do not. In a luxury neighborhood it would make sense the shoppers are looking for experience more so than low cost.
(I am less sure why this happens to such a great extent in large US cities vs elsewhere, vs more of a balance of shops, but it does seem to.)
So it may just be that where they are from, the concept of “luxury neighborhoods” which do not need to cater to the poor and disadvantaged populations does indeed exist, due to segregation factors