Why shouldn't personal data like age, race, or gender be included on a résumé?

Personal details section in resume is generally the easiest one to write. When you are writing this section, it should be limited to the relevant details. Employer cannot ask you the details like your age, gender, race and religion. Hence, you can provide the other important details related to the job in your CV. This section will evenly contribute in improving the quality of your application.

Because you do not want to release that information to do not know.