Only three Devonian tetrapods are known. None has five toes. They bear, respectively, six, seven, and eight digits on their preserved limbs. As with so many other actual facts of nature, we point to the obvious intelligence of the designer. For example, look at the many types of cars that have four wheels.
Designers utilize a proven principle.
We believe that all the creatures around us were created by a super-intelligent Designer. He utilized whatever design he considered best for each given situation, and in a great many cases we've only begun to recognize the tremendous complexity shown in those designs. 14-18, wrote about the broader principle illustrated by this question of pentadactylism. Our interpretation is determined by that knowledge-system. This is why evolutionists insist that the concept of a God is only religious, and thus outside of the domain of science.