If time travel is possible, why has nobody visited us from the future yet?

I was was thinking about this. If time travel is theoretically possible, I would think sometime in the distant future (hundreds, maybe thousands of years in the future) human beings will have developed the ability to travel in time. Then if future humans will inevitably figure out how to time travel, why have they not traveled back and told us some of the mysteries of the universe?

I was thinking perhaps time travel is being kept under extremely strict restrictions by a group of powerful individuals to prevent people from messing stuff up.

But then again, there is the possibility that human beings wipe eachother out before we reach that level, and that is why we have had no people from the future.

I am not even sure what my purpose of posting this is.. I just love the idea of time travel :D