Like in plants & trees, physical growth in humans too happen till a stage in life. Then the process of decay kicks in. Like in plants & trees, only during this decay process, the fruits of the labour during the growth stage is formed and made available . And the decayed physical components of humankind merging with its origin, ie, SOIL, ensures the sustainability of this wheel of Life.
BTW, the fruits are meant to be cherished to span new wheels of life, continuing the tradition.