Why it happens
Cakes dry out mainly when they're overbaked, because the heat drives off moisture and the crumb sets too firmly. Too much flour, too little fat or sugar, or an oven that runs hot can also give a dry result. Sugar and fat both help keep cakes moist, because sugar holds on to water and fat coats the flour and limits gluten development. Once moisture has baked out, you can add some back from outside, but the texture won't be the same as a well-baked cake.