SPOILERS

Cersei's army had mostly fled before the unsullied arrived to take the castle. They took the food with them. The idea was to let the unsullied take it then starve while Cersei's army moved to take Highgarden which they believed to be strategically more valuable.

Why is it that the food that feeds the seven kingdoms comes only from the Reach? They are so unprepared for a long winter. With or without white walkers.

Much of the rest of the country was already laid waste by the wars.

(Like the farmer and his daughter in the riverlands, who starved to death after the Hound stole what money they had).