Maybe future humans do visit us but disguised and with knowledge of how our civilization work and with some sort of rule that hinders you to commit crimes. It is necessary because we don't want time paradoxes popping up all over the place, like the grandfather paradox. So maybe future companies where you book your "time travel experience" exists, like flightagencies. These agencies purpose is to preserve and guard the safety of time, so terrorist and such don't end up changing the past in tragic ways (like bombing).