If one kidney can function just as well as two, why do humans have two kidneys? The cost of growing two kidneys must surely be quite high, especially since one kidney is all that is really needed.
One of the easiest answers to the question; "Why do we have ____?" is "Because our ancestors did". This is not trivial or flippant, as it is a significantly important answer that is so often overlooked. Humans are members of Bilataria; a deep branch of the animal tree that is characterized by bilateral symmetry. Humans are typical of other vertebrates in having two kidneys. Selective pressure to reduce to one of two organs is typical in snakes, whose body form favors reduction. But even snakes have two kidneys. Natural selection does not reduce features we don't need unless such a reduction would increase fitness.