Movie time travel should never be taken too seriously as it is just a plot device rather than science. The fact that these things are not happening all the time suggest travelling back in time it either impossible or really hard. We are not over-run by time-travelling tourists, as far as we know. I've seen too many cases where someone goes back in time to invent time travel. They just did something like that in Loki too. How does the loop start? This stuff can make my brain hurt.