There are at least three main reasons why sexual harassment in the workplace is not always reported.
First, there is the fact that women know that many claims of sexual harassment will end up being their word against the word of the person they are accusing. Many cases of sexual harassment have to do with words and actions that are not witnessed by others. This means that a woman who comes forward cannot be very certain of being believed.
Second, women have to worry that their claims will not be taken seriously. Even if the harasser admits to his actions, they may worry that they will be told that his actions are nothing serious. They may be told that they should not be so sensitive about everything.
Third, women may well worry that reporting sexual harassment will harm them in their careers. In many jobs, people are selected for advancement and promotion based on intangible factors. Women who complain of sexual harassment may feel that their superiors will consider them to be sensitive whiners who do not have what it takes to advance in the company.
For these reasons, many instances of sexual harassment in the workplace are not reported.