"Hindsight is 20/20" because perfect eyesight is 20/20: https://www.aao.org/eye-health/tips-prevention/what-does-20-...
(20/20 means "person being tested sees as well at 20 feet as standard vision at 20 feet.")
This isn't obvious; consider that having 3,000/3,000 vision ("I can't read text all that well when it's half a mile away") says pretty much nothing about how clearly you can see things that are 20 feet away.
Nearsightedness and farsightedness are both characterized by "normal vision" at certain distances which becomes markedly worse at certain other distances.
3,000 feet is quite different (for one thing, atmospheric effects start becoming relevant).
That is how the saying is used, but oddly enough it's pretty much the opposite of what 20/20 vision means.
> Some birds of prey, such as hawks, are believed to have an acuity of around 20/2; in this respect, their vision is much better than human eyesight.
The scale of how good vision can be keeps going and going indefinitely (presumably until you get into physical limitations to resolution related to wavelengths). While 20/20 is considered normal and not in need of correction, it doesn't mean it's the best vision observed among human beings, or the best that's physically possible (for human beings or other animals).
https://en.wikipedia.org/wiki/Superman#Powers,_abilities,_an...
> I thought 20/20 meant you don’t need glasses or any other aids to see.
No, of course not. Try looking at Saturn at night. Then try looking through a telescope. Odds are you'll see it better with the aid.