[brianpan]

The answer to pretty much every biological "why" question is: because it reproduced. It seems simplistic, but really, a thing is here and alive because its ancestors reproduced.

Your version of the question has surprising perspective- I think you are asking what the "it" of the plant is. That's an interesting personification of a plant. I think it points to the fact that plants may be safer underground- for anchoring, for not being eaten, for getting shielded from harsh elements.