Getting something crispy requires getting all the water out of it. By definition, you can’t get all the water out of something by cooking it in water. When water hits 100 C, it boils, but it will never get hotter than that. Oil will get hotter than 100 C, so it will make the water in the food boil out. When you are frying something, you are really just cooking all the water out of it.
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