To replicate the soft, creamy mouthfeel of dairy fat, manufacturers blend cocoa butter with softer plant fats like coconut oil or shea butter. Cocoa butter is naturally hard and brittle, whereas milk fat is soft at room temperature. If you used only cocoa butter, the milk-like chocolate would be too hard and waxy, adding softer plant oils lowers the melting point, ensuring it melts smoothly, just like real dairy chocolate.