Because men can easily rank women in terms of attractiveness, and calibrate their strategy based on previous experience. Men operate in a very systematic way. E.g. "my success rate with 9s and 10s is very low, less than 5%, so it must mean I'm aiming too high. With 7s and 8s, I get rejected about 80% of the time, so I'm still probably aiming too high. Sweet spot seems to be with 5s and 6s, so I'll go after them."
I've never followed this strategy, although I certainly recognize its charm. It's about rationalizing the world and trying to make sense out of things not in your control. Nevertheless, a much better strategy is to only go after girls you find really attractive, even if that does mean a higher rejection rate.