One reason is because the geometry of nature is different from that of our tools. Its self-similarity and fine detail are derived from growth processes that are very different from the working process imposed by drawing programs. This mismatch makesit difficult to create natural looking artifacts. Drawing programs provide a palette of shapes that may be manipulated in a variety ways, but the palette is limited and based on a cold Euclidean geometry. Clouds, rivers, and rocks are not lines or circles.
Paint programs provide interesting filters and effects, but require great skill and effort.
Always, the details must be arduously managed by the artist. This limits the artist's expressive power. Fractals have stunning visual richness, but the artist's techniques are limited to those of choosing colours and searching the fractal space. Genetic algorithms provide a powerful means for exploring a space of variations, but the artist's skill is limited by the very difficult ability to arrive at the correct fitness function. It is hard to get the picture you wanted.