We can figure out how trees grow on this planet and how they came to be on this planet but what or who even CAN decide why they grow on this planet?
Also, this: http://www.imdb.com/board/bd0000130/nest/263626706
Or we can take these message boards being shut down as another example.
We can say how it is being shut down and we can say how it came to be being shut down either with short and sweet "The people in charge wanted to shut it down and are doing so." or in great detail reaching back through human history to before atoms and photons existed but as to why they are being shut down, who or what even CAN decide why anything on this planet is happening the way it's happening?
The people in charge can give their reason for shutting the message boards down and you can be satisfied that answers why it's being done but that's no different from asking "Why do trees grow on this planet?" and being satisfied with "Because humans grow them for food."
Also, this: http://www.imdb.com/board/bd0000130/nest/263626706
Trees grow on this planet because multicellular life in the oceans mutated the ability to produce sugar from sunlight, leading those life forms to thrive close to the surface of the ocean. Eons and eons of continued reproduction and further mutation led to the earliest land-based plant life, which then further evolved as competition for sunlight led to taller and taller plants winning out in the process of natural selection.
That's both the "how" and the "why." DNA mutates. Some mutations are beneficial.
"Why" is really a useless question, anyway, as it ascribes human intention to random actions. Why are Republicans statistically less intelligent than everyone else? They just are; the party evolved into that through the desire to keep women and minorities from having rights.