How do you know that such things actually havent happened? Lets say that your parents would never have met eachother without some effect of the time traveler.  Guy directs where your father works, or where he is !@#$%^&*igned, or which car he rear-ends and meets the insurance woman, junk like that.   Or even, someone from the future does go back in time and ruin it there.  Maybe the guy is from 3031, and -*BAD WORD*-s up time in 2031.  That wouldnt have effected us would it?   Or even, maybe time travel is protected and restrictred, no one goes back in time to -*BAD WORD*- anything up!  Just talk to some guys down in poles in some dust cloud.