Ancient Aliens?

I am not particularly convinced, however the idea of extraterrestrials visiting Earth in ancient times and shaping our beliefs and culture IS fascinating. If we were visited by ancient aliens...where are they now? If they were so technologically advanced...why has sophisticated technology only come about in the last two centuries? Why are there only crude stone depictions left over as evidence of these 'ancient aliens', wouldn't there be more sophisticated electronic artifacts left behind?