> To this range of answers, I'd add "because God created us (because of reasons outside the scope of this answer)"
Yes, but which god, and why does this explanation get any greater credence than Douglas Adams' writing "the Jatravartid people of Viltvodle VI believe that the entire Universe was in fact sneezed out of the nose of a being called the Great Green Arkleseizure. The Jatravartids, who live in perpetual fear of the time they call The Coming of The Great White Handkerchief, are small blue creatures with more than fifty arms each, who are therefore unique in being the only race in history to have invented the aerosol deodorant before the wheel."
The religions of the world disagree with each other on the "why", and none of them seem to be interested in demonstrating the veracity of their theory. This is why science generally ignores the "why". No-one has come up with a reasonable way to show, one way or another, which theory may be truer (or more aptly, falser) than any other, and therefore the question currently has no meaningful answer. If this changes, it will show that all, some, one, or none of the current religions are correct, but thousands of years of discussion haven't produced anything close to solving this problem, and this discussion has been going on since before Akhenaten.