Why should the farmers not leave the land of farm bare after a harvest?

Actually they often do leave the land unplanted. Some crops remove minerals from the soil and the land needs to sit for a bit unplanted. It is called fallow land when this is done.

Answer

Farmers may find that planting "cover crops," which are crops grown for a short time and then plowed under is more advantageous than bare soil. Depending on the crop chosen, doing this can add nitrogen and other nutrients back into the soil. This may also prevent loss of topsoil due to wind erosion.