The prehistoric realities were harsh and nearsighted individuals had little chances of survival and procreation (natural selection). In the modern society perfect eye-sight is not as important for survival as in the prehistoric society, thus more tolerance to its greater variability.
The timespan is too short for evolution to have had a considerable impact.
I am not talking about evolution as in developing new species but about survivability of naturally occurring variations. Modern humanity timespan is not enough in developing new genetic traits but long enough to alter effects of natural selection. BTW, due to advances in medicine nowadays we have children born and survive into adulthood with medical conditions that were terminal just 100 years ago. Survival variability (in developed world) is much broader that it was in recent historic past (e.g. middle-ages). This is a plain fact.