Because its huge part of society and so we have to try and encourage people to interpret it as loose metaphors, rather than the nutcases who interpret it literally and try and kill people or create laws based on it - thats why.
Your understanding of gravity and orbits is ridiculously oversimplified - go and read how these mechanics work and then you will your answer.
Human nature is still nature.
If (insert apex predator - eg lions) eat another species into extinction then its nature. If humans destroy another species through their behaviour then its still nature.