Well, the fact that we have any eye-site at all, is a testament that nature selected VERY STRONGLY towards ever increasingly better eye site.
Even those people who do need glasses, have incredible eye site, compared to our very ultra-ancient ancestors (many of whom were creatures that were completely blind if you go back far enough!).
But still... the question remains: why do so many people in our population need "corrective" eye glasses?
The answer is obviously that for some reason, many of our distant human ancestors did NOT need eye glasses to survive to reproductive years.
Since we are a tribal species, their vision deficiencies were offset by others in the tribe with better vision, who performed vision critical tasks.
If all humans needed 20/20 vision, or better, in order to reproduce, then within 1 or 2 generations those with less than 20/20 vision would have died out... almost IMMEDIATELY.
There would probably be a huge die-off of 60 percent of the population or so... leaving the next generation descended from the 20/20 visionaries.
Similarly I'm guessing there are VERY FEW eagles or hawks with blurry vision, because their species depends MUCH MORE upon super-vision, than our species does.
So eagles with bad vision probably die off very quickly, since they don't have tribal culture to support them.
Finally, you could expand your question to ask:
Why do 100 percent of all humans need extreme vision-correction, in order to see the infrared and ultraviolet spectrum?!
Hardly anyone can see infrared light without infrared goggles, and yet it seems like it would be highly advantageous to see in that frequency (which is very close to visible light).
So why can't we see in infrared frequencies as well?
Well, quite simply because nature hasn't yet got around to us evolving towards that part of the spectrum, because our current vision frequencies are GOOD ENOUGH.
And so it would seem that those with less than 20/20 vision, still have vision that is GOOD ENOUGH for survival in a tribal-species-culture.