Because most of the time there is greater variation within a group than there is between groups. If you use group statistics to judge individuals you will be wr

Because most of the time there is greater variation within a group than there is between groups. If you use group statistics to judge individuals you will be wrong more often than if you don't. But beyond that, we as a society have decided that in many cases the benefits of using group based heuristics aren't worth the damage done to individuals within groups who are judged negatively. Imagine you toss out all resumes with non-white sounding names because white people are statistically more likely to have x, y, or z characteristic. Even if there is a correlation to some characteristic that you are looking for (more likely to have a college degree, statistically higher income etc...), the damage you do to the individuals in the groups you are tossing out far outweighs any benefit you receive.