That is a very good question. Why do they keep inserting people back into the Matrix to continue the slavery? The easy answer would be to preserve the environment of the Matrix; that is to say, it would be necessary for people to see people being born and aging into adults to accept the programming of the Matrix.
Which brings me to the next question: why not just use artificial intelligence to simulate human beings? This one has no easy answer.
However, notice that we have not seen a single program age in this movie. The oracle, Seraph, the Merovingian, and Agent Smith could be ten years old or one hundred and ten, yet they would look the same day after day. Perhaps humans on the real Earth lack the programming to create an artificial human program that can be born, age, and eventually die.
Yet this is not to say that it could not have been attempted. Remember Troy (who I earlier suspected to be a ship captain)? He paid Neo a sizeable amount for a disc that said, "AI Disc" on it. Now, Troy made various references and hints about who Neo was; for example, "Hallelujah. You're my own personal Jesus Christ," and "It seems like you need to unplug , man." Perhaps his reference to Neo as a savior was his hints that he was a Zion ship captain who knew Neo may be the one.
Or, troy could be a program or a human working for the humans on the surface. The disc said "AI Disc," and may have been a component that was necessary in creating the AI for a human program that could grow and age. If so, Neo would not only be the supposed savior of Zion, but of the humans on the surface living behind a wall of lies and robots they control.
This is a theory I myself did not believe at first (humans vs. humans rather than robots vs. humans). However, it seems like everything with this theory is falling into place, most recently with what I explained about Troy earlier in this particular reply.