In the same way a book is representative of its author. You seek a denominative equality between the word "intelligent design" and "science" when the two are hardly synonymous in the first place.
Evolution is not science in the strict sense either. It is a hopeful, multifaceted reconstruction of history based upon extrapolations and conjecture from the physical world. Nevertheless it does engage science in support of its assumptions, not unlike astrologers who invoke the stars in support of their predictions.
Science is "the observation, identification, description, experimental investigation, and theoretical explanation of phenomena," none of which can take place without intelligibility, intelligence, and design. Intelligent design is one of many deductive and inductive approaches to science. It is not to be identified coterminously with science.
You apparently live under the delusion that science can take place devoid of philosophies, basic assumptions, and shaping principles. That cannot happen any more than one can enjoy life as a corpse. Just because evolution operates under basic assumptions is no reason to say outright "it is not science." But it ought to admit honestly and outright what those assumptions are.