The "why" is a whole world of debate in and of itself, and is in fact a problem with the language used to convey scientific understanding. From Wikipedia: Tele

The "why" is a whole world of debate in and of itself, and is in fact a problem with the language used to convey scientific understanding. From Wikipedia: Teleology (Greek: telos: end, purpose) is the philosophical study of design and purpose. What happens is that ideas are often conveyed using teliological phrases, such as "the sparrows migrate to africa in the winter month so as to find abundant food sources and rear the next generation..." or "testicles reside outside of the body major in order to benefit from a reduced temperature...". The problem is that these make sense to us, although they are _very_ incorrect. The teleological concepts of "so as" or "in order to" are the whys and they are very tricky beasts. So, for utmost correctness one would prefer to say: "the sparrows migrate to africa in the winter month finding the abundant food sources needed to rear the next generation..." or "testicles reside outside of the body major benefitting from a reduced temperature..." It's a very subtle issue, and one that is compounded by the popularity and ease with which we talk about biology and evolution in particular. Ernst Mayr wrote some spectacular essays on the subject, he's one of my heros so forgive me for the blatent plug! He claims that a formal understanding of the errors of telology must permeate the sciences, and most especially biology (he was a philosopher of biology), so as to clear up the ambiguities that can lead one to think there is an end or purpos (telos) to evolution. Even now it is still quite common for people to think of it in terms of progress and constant betterment instead of just simple and blind adaptation.