Because of course they are. It's the only explanation for why our basement is filled with 8-tracks and lava lamps.
But in all seriousness, our parents have discovered a way to travel through time and have been visiting ancient civilizations.
They're not just visiting, though. They're actually trying to prevent the invention of the microwave.
Our Parents Traveled to the Renaissance (and got lost in a gallery)
Our Parents Visited the Hippy Movement (and got stuck in a VW Van)
Our Parents Tried to Stop the Roman Empire (and accidentally started a battle)