Because it's had millions of years to converge on a near-optimal solution for the environment that it's in. In fact, this is true for all organs within all extant species.

Nature is far more advanced than technology, and I don't believe that we'll ever quite be able to one-up her. Most efforts to do that have seriously harmed us.

We think we know everything, but this dangerous hubris can destroy all planetary life in the blink of an eye.