The main reason is because the interiors are dehydrated. When they are initially cooked, the starches in the interior trap water and give fries that fluffy texture in the middle. When they cool, that water leaches out of the interior into the exterior. So you get a gritty interior and soggy outside, and no amount of reheating will fix that.
Reheating in the microwave exacerbates the issue. Microwaves primarily work off of heating up water molecules. Since the interior lacks them, the interior stays cold. But microwaves don’t get hot enough to actually boil away the water to make the outside crispy again. So while the outside will get hot due to the presence of water, it stays soggy and gloopy.
Using an oven or air fryer can help, as it actually heats away the water on the outside and can return some of the crisp, but the inside really can’t be fixed.
Frozen fries avoid this issue because the original moisture inside the fry is still there, and can’t escape because it’s been frozen in place.
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