So there's two types of food deserts: rural and urban

Rural food deserts are typically some small dying town populated by overworked commuters (who primarily work in an agriculture or agriculture supporting businesses, since that's what exists near these towns) and tax- and price-sensitive retirees. It's a small town of less than a thousand people and everyone just drives to the Walmart the next town over because that's what they have for options now.

Urban food deserts are typically in poorer sections of the city (areas whose poverty was reinforced by historical practices of redlining and broken window policing for example) so any businesses that do sell food tend to be dollar stores which due to their margins can't afford non-shelf stable food, or gas station convenience stores which both can't afford non-shelf stable food and might not have the space to dedicate to some fresh produce. Grocery store chains will generally aim for locations with some wealthier clients since they'll spend more per trip, and they fear that opening locations in less affluent areas will lead to more crime.

Some cities have responded to food deserts by directly subsidizing the opening of a grocery store, or subsidizing the availability of fresh fruit and vegetables at existing stores, but that requires a level of civic engagement that not all cities are willing to put in, and more affluent citizens may turn their noses up at such neighborhoods and just want them to decay out of existence (which isn't going to work) or worse they try it and find that most of the people who could be supporting a small local grocery store still go to the big grocery store/Walmart that they have to drive to instead, so the local grocery store goes out of business despite the subsidy

Because the ones that "have", don't care about the ones that "don't have".

And the ones that do care ,can't help everyone.