I have never once heard someone try and claim Amazon is why food deserts exist...

Only a magically perfect network would do that. There's no force to "correct" food deserts.

OTOH, Amazon grocery delivery in cities does do a LOT toward reducing their impact on poor people (who are not-poor enough to pay for Amazon delivery) (which might be cheaper than bus tickets). Hate on Amazon and Bezos all you want; some good comes out of it.