Distances. Our fastest spacecraft onky flies 11700mph. It would take years just to get to some planets in our own system. We could launch an infant and theyd grow old and die before reaching points outside our system. Currently faster than light travel is impossible except in theory. Then there is theres the problem of how to supply one or more astronauts for years or a life time. And currently interspace communication over great distances is generally impossible as it takes years for a message to travel to and from earth as sound travels more slowly even than light.
Great distances between stars. Even at the speed of light the nearest star is a little over 4 years away and we are no where near being able to go that fast. So, we'll have to be satisfied with interplanetary travel for the time being.