Though I don't know if I'm fully correct in my answer. I would say it is because we have not created a fast enough way to travel that distance and we have of course not made the technology to inanimate a person for long periods of time while keeping them completely in tact. Since humans can only live to a certain degree and it takes far too long to reach a planet in that solar system. While also the resources need for human survival would run out before making it as well, most likely.
