Because it is not the nature of non-intelligence and non design to produce intelligible or purposeful matter and functions. Reasonable people, like reason itself, expect entities to act according to their nature, and also expect, when things act according to their nature, they will be consistent. It is inconsistent with the concept of intelligent design for unintelligible substances and events to develop.
That is not to say one may not dismiss intelligible matter, its organization, and its functions as merely having the appearance of being intelligently designed. Evolutionists do that all the time, but they do not have a scientific argument. It is a personal philosophy whereby the evidence is waved away in favor of personal predilections.
Why is it more reasonable to believe what is written in a telephone book than to disbelieve it?