We don't know that they haven't.
If time travel to the past is possible, you might want to reduce the visibility of your presence for a couple of reasons. Firstly, it's conceivable that you might mess up a timeline (not necessarily your own, but you might consider preserving alternate histories to be just as important). Secondly, there is every chance that a traveller to our time would be highly sought after by government agencies desperate to "interview" them about technological advances, economic and political forecasts, and territorial/military intelligence. There are probably other reasons that wouldn't even occur to us stick-in-the-mud unitimelineans.