can somebody ELI5 how "food deserts" work (or don't) in the USA?

I’ve heard this term a couple of time but never actually looked into it, and it is such an alien concept to me right now. I apologise in advance for sounding dumb here.

I can understand slums and favelas having a harder time getting access to fresh food, but how come entire government-recognised and incorporated neighbourhoods with electricity, water and all those more complex services can’t have small grocery stores for basic healthy things like rice?