Because our laws don't change people's behavior or have the ability to prevent anyone violating them otherwise we'd have no crime just as an example. What they do is
- Explain the prohibited behavior
- Outline the category of the crime (felony, misdemeanor, traffic infraction, etc. and
- State the punishment for violation of said law
For a long time African Americans had no recourse at all for having their rights violated. When the slaves were freed via the Thirteenth Amendment people of African descent though no longer slaves, were not considered citizens of the United States under the existing law, therefore they still had no rights to many of the things we all have the right to today, including the right to bring a lawsuit to address the legal wrongs done to them. That's what it means to have no rights and it is racial discrimination when one race benefits at the expense of another.
So to answer your question, even though all of these laws were passed the majority of the country still openly practiced racial discrimination thus necessitating that it was "put in writing" to
"not discriminate against any employee or applicant for employment because of race, creed, color, or national origin" and "take affirmative action to ensure that applicants are employed, and that employees are treated during employment, without regard to their race, creed, color, or national origin"