Maybe they don't want to 'come back'...
Personally, I don't believe in the barrier of light. I believe our understanding of physics has not yet reached a point where we can control how we manipulate matter when approaching (or passing) the sped of light.
As far as the "time machine" inventions, well, whose to know if one hasn't been invested in some far off future and not already been used? None of us here would KNOW if world event's were altered by a future presence. It would all seem like history as normal for us.
This being Christmas and all, I would most certainly want to go back and talk to Jesus Christ. In my mind he has allot of splainin to do. Just think of the possibilities!