I agree that the purpose of the constitution is meant to secure the liberties of people, but to say that we already have them from a modern lens disregards the meaning of "rights" throughout time. The constitution was a product of its time; the 3/5 compromise was needed to pass in order to get southern states to ratify it, thus constitutionally allowing slavery to exist. Freed blacks were largely discriminated against during Reconstruction despite the 13th, 14th, and 15th amendments had already passed, including other acts such as Grant's Enforcement Act.
Because they were not clearly defined, they were subject to loophole exploitation. At that time, who was to say that they ever had rights? Decisions in Plessy V. Ferguson and Cruikshank undermined them and failed to protect African American rights, hence the existence of Jim Crow laws. By definition they were meant to have rights, but what use is it if they are unable to exercise them? Therefore I don't wholeheartedly agree with the concept of already having rights if it contradicts historical accounts.