They work so well because they're also "baked in" the training run of the model. The concept is simple, but training it to actually use it unlocks the "wow" factor. (using cc with other models, not trained specifically for this, reveals how big training is)

They also highlight this in their docs, you don't need to teach claude about skills. Or to write new skills. It "knows" what skills are, how to use, edit them etc.