The reason is simple survival.
Human vision developed to deal with spotting hidden / camouflaged prey and predators.
You better be able to make out the face of that lion hiding in the grass or you are dinner.
So your brain constantly scans for anything looking like a face, a face of anything human, animal, bird, fish etc.
Most folks probably don't know how to tune out their busy mind and concentrate on what the automatic brain is doing, but many people can do so, and as such you can consciously access all the pattern matching that your automatic brain is doing.