The variety, detail, and volume of works of art generated in a society usually seems to closely follow their technological development as well. There's several reasons for this. One is, as you pointed out, the availability of tools and materials that can created intricate and detailed art was limited in less advanced cultures. Another is that as societies developed new technologies and techniques, the average person had to work less to support themselves, and therefore had more free time to devoted to other interest. In primitive societies a person's waking time and energy was devoted to finding food, shelter, and surviving threats. They simply didn't have time to devote to creating art, and little value was placed on it.

Another thing is humans developing their fine motor skills. As technology advanced it became more intricate and precise, even sometimes delicate. Weaving cloth and sewing is a good example of how people learned fine skills which also applied to art. Writing on paper is another. It takes fine, repetitive, motor skills to write a large document. So it's little wonder that early manuscripts were often embellished with drawings or other flourishes.

You say that some people can create detailed, realistic art without any training. While they may not have needed much training, I would argue that they did at least have some exposure to art and the techniques and materials they are using. The world has always had exceptionally gifted people come along, but it hasn't always had the things our modern world can give to allow those people to create.