Why is 20/20 used to represent "perfect" vision?
Actually “20/20 vision” is not at all “perfect vision.” Perhaps an eye doctor told you something like that at the risk of over- simplification.
20/20 (or 6/6 in metric) simply means that distance vision is “normal” in terms of the size of objects discernible at 20 feet or beyond (acuity). It is quite possible to see better than this (i.e. with even better acuity) such as 20/16 or 20/12.5.
Furthermore, the term “20/20″ (or “20/anything”) says nothing about one’s ability to see at near, e.g. for reading.
There is far more to good vision (much less “perfect vision”) than acuity: contrast sensitivity, visual field, color perception, et al.
So, better to think of “20/20 vision” as normal vision, not perfect.