Why do you think this is true?
Pulsars are a type of star that radiate a rhythmic pulse to a regular beat. It was originally believed to be the product of "little green men" in origin because its discoverer didn't think that such a thing could be natural.
Klerksdorp spheres were also originally thought to be man-made, but later turned out to be wholly natural.
Besides, this "design inference" is clearly not a standard that you actually hold. Rocks, sand, tides, and much of the rest of nature doesn't look "designed", but you believe that it was. It's not that things look designed, but rather your presupposition that they were that drives your belief.