Since a Time Machine has yet to be created. When one does exist we may not be able to go back to a time when it didn't exist in order to not create a paradox by messing something up were we prevent it's existence.
A time traveler coming back now could very well cause a paradox. They could know that no time travelers came back to now and if they did that would cause a paradox. Or some government may steal the time machine and use it to their own purposes which would cause a bootstrap paradox.