Because plastic hadn't been invented yet. (Just kidding.)
There are four layers of existence: Inanimate objects, plants, animals, and humans. Each level takes the level of life which is below it and elevates it. A plant derives its nutrition from the ground, elevating the ground and incorporating it into a higher level of existence. The same is true of all the levels. One reason we were created from the ground is to remind us that we can constantly strive to elevate every aspect of existence, even the lowly dirt.