Yes, traditional artists buy their paints and canvases, and 3D artists buy props and textures. But if a traditional painter buys a canvas with "painting by numbers" on it, one might regard the end product as less than a work of art. Some parts of the 3D creation process (for instance lighting) are an integral part of the composition. Putting in a ready-made lighting set means that the artist won't learn an important skill, and worse, will get the idea that lighting is something extraneous to the composition.