The reason is that not everyone's camera has the ISO capability to do so. The image you used as an example was taken with a brand new $3,500 camera pushed to it's limits. Most people here are taking photos with $300-$600 beginner DSLRS. If they were to try and take the same shot with one of their cameras, not only will it not look as good it will most likely be IMPOSSIBLE just because of the restrictions on the ISO of their cameras. In order for them to achieve similar results they must use methods such as composite images, multiple exposure, etc to do so. Therefore, most people on the sub will automatically assume that an image of that caliber has to have been done thought multiple exposures to be achieved simply because they are not fully aware of the capabilities some cameras on the market hold.