Answer:
Humans haven't visited any other planets because we haven't developed the science and technology to do so. There are many things we must accomplish before we can go to another planet. There are also a few circumstances that may complicate a trip to another planet, such as the time that it will to reach said planet. We also must think about the conditions on said planet like the atmosphere for example. What if people arrive there and find out its too cold or too hot?, what if they get their and realize there is no breathable oxygen? I doubt that this would happen but it is still something we must thing about.