hoveringsombrero | The answer is statistical improbability.

" we find a transparent pebble washed into the shape of a crude lens by the sea, we do not conclude that it must have been designed by an optician: the unaided laws of physics are capable of achieving this result; it is not too improbable to have just "happened." But if we find an elaborate compound lens, carefully corrected against spherical and chromatic aberration, coated against glare, and with "Carl Zeiss" engraved on the rim, we know that it could not have just happened by chance. If you take all the atoms of such a compound lens and throw them together at random under the jostling influence of the ordinary laws of physics in nature, it is theoretically possible that, by sheer luck, the atoms would just happen to fall into the pattern of a Zeiss compound lens, and even that the atoms round the rim should happen to fall in such a way that the name Carl Zeiss is etched out. But the number of other ways in which the atoms could, with equal likelihood, have fallen, is so hugely, vastly, immeasurably greater that we can completely discount the chance hypothesis. Chance is out of the question as an explanation."