Just to hazard a guess, one could assume it has something to do with the way our brain categorizes different "contours" and patterns in the visual fields as being faces or fingers or really anything more than just lines. Take looking at a cloud, and seeing a vague shape of a face and recognizing it even though it's quite clearly water vapor. Now given that a drug like LSD makes you more suggestable, and that it's already overloading your neurons in such a way as to cause you to see fractal patterns where none exist, then it seems logical that it would also overload the neurons relating to matching those patterns with known objects like faces, causing you to falsely associate your hallucination with the perception of a face or entity.