Pteridophytes grow above the soil as they are plants that have leaves with chlorophyll and can produce their own food by the process of photosynthesis. This requires sunlight as a source of energy which is not available below the ground.
The species of plants that fall in this category have structures made of single elongated cells, called rhizoids that serve the purpose of roots and absorb water and other nutrients essential for the growth of the plant from the soil. The absorption is done by capillary action unlike that in the roots of more advanced species of plants. But the formation of rhizoids allowed plants to move from living only in the water to living on land. The rhizoids also anchor the plants to the ground that enables easy growth.