Maybe because hydroponic gardening works better. People still use soil because it is available and they do not know too much about hydroponic gardening. When you use soil by planting a plant you are enriching your soil. This also breaks up the rock to make it easier to grow plants. They also leave nutrients behind after they decay.
Soil can work probably just as well as hydroponic gardening.
There are three different types of cultures in the hydroponic gardening method. In the gravel culture people use gravel, water and sometimes chemical. The gravel culture supports the plant so that the plant is suspended above water. In the water culture people often use plain water. This method is helpful because water supplies some of the essential elements that plants need to grow.