Clearly, when individuals select themselves to a sample, the significance testing procedure is flawed and conclusions about the variable cannot be drawn. Suppose you wanted to know whether men or women were more conscientious and you have available applicants for a promotional position. Conclusions would not generalize to women employees from your organization and men employees from your organization. Any conclusions would be specific to the applicants only. You certainly couldn't make generalizations to women and men in general.
Applicant groups are always self-selected.
Additionally, the populations they randomly select themselves from can hardly be compared to any more general population. Therefore, conclusions do not generalize beyond the specific individuals. When you observe statistics from applicant groups, these cannot be generalized to other groups. Not to women. Not to Hispanics.