Literally: Because the earth was of habitable climate and makeup, which allowed for the slow evolution from single cells to complex creatures. One branch of this evolutionary tree became people after billions of years. Hence, us.
More Philosophically: The reasons for are existence are given by a myriad of confident and false religions, but if you wish to subscribe to one of them, I would suggest you watch Carl Sagan's
Pale Blue Dot speech first, as I believe it adequately describes our lack of spiritual purpose in this world, while also showing that a "higher purpose" is not needed to live a full and enjoyable life.
also, inb4 Kalistean highjacks the thread to make it retarded again