I am not saying that traveling back in time is not possible, just that people from our future can not exist on our timeline.
First off, I doubt time travel is possible due to the numerous paradoxes and a vast amount of unknown/unknowable factors. Worst case scenario, it would cause a black hole so vast it would swallow the universe. Best case scenario, nobody would know i it even works.
Best case scenario: When the person travels back in time, his arrival creates a branch off the original timeline. Basically he can't exist on the original timeline so a new one has to begin from that point in the past. Seeing that people from your original timeline never hear from you again, they believe that either it sent you to the wrong place/time or that you just got destroyed.
The reason why we wouldn't be able to exist on the original timeline is it would create a contradiction loop in time were two opposing thing would have to co-exist.
Hypothetically, if person A from the year 2020 was to travel back in time to kill Hitler as a child and succeeded, the world would never know what he would have become. This means that person A in the year 2020 would have no reason to go back in time so never does, at least not to kill Hitler so that would mean that time would revert back to when Hitler rose to power. Again, this causes 2020 person A to travel back and the loop continues. See the problem? In some sci-fi, they just try not to make any big mistakes like killing an ancestor or doing anything big. Problem is, just being there makes a contradiction in the time line. This means that you can not exist in the original timeline.
About the only way we could have time travelers among us is if they came from a different timeline and caused this one. Again, I doubt they would send people when sending other thing would show no results making it seem like it does not work.
Note: I was going to explain more but not enough space and time. Lol
I'm only putting this here because the site requires you to post your side of the opinion when you make these. I really don't know though, I wouldn't rule it out but time travel just seems impossible! But electricity and computers would probably seem impossible to people from 1,000 years ago so again I'm not sure.