Each part of the plant has a specific function. A root's function is to take in raw nutrients from the ground. The shoot's function is to eventually start growing leaves on its stalk, so the leaves can collect light and start photosynthesis. Photosynthesis is a process that uses light to convert the raw nutrients absorbed by the roots into plant food.
As the shoot grows, it starts to form leaves. Leaves are light collectors. Roots, on the other hand, grow in a different manner. New roots form on the main root, and these new roots start to grow out. The growth looks like a capillary system, if all the soil was removed. Shoots do not grow out in a capillary system, but rather grow off the main stalk, or even form main stalks themselves.
Differences exist in how roots and shoots use nutrients. Research Botanists W. Wanek and S.K. Arndt ran extensive studies on how soybean plants use nutrients, particularly nitrogen. They discovered that different different parts of the plant concentrate different levels of nitrogen. Studies demonstrated that nitrogen-rich soil leads to a decrease in nitrogen levels in the shoots, but an increase in the roots. The reverse is also true. Decreasing the nitrogen levels in the soil results in decreased nitrogen in the roots, but a concentration of nitrogen in the shoots.
Roots and shoots are made up of various types of cells. A root's cells have thin walls, and by and large the "wood" is weak. A shoot's cell structure is completely different. For many trees, the shoots grow eventually into wood. Wood has a tough, thick cell wall. You can build things out of wood because the cell walls are very thick, and resist bending. Roots, with thin flexible cell walls, are not suitable to use as a building material.