When Aliens Arrive On Earth - They Just Might Be Bio-Robots?

I've often thought, why would a more advanced alien civilization want to travel all those millions and millions of miles, taking decades, even centuries travelling at warp speed, just to come to earth. 

For the little bit of gold? I don't think so. If a civilization were so advanced to travel those huge distances they probably already know how to turn basic metals into gold, alchemy (the transformation of matter). Wouldn't you think they'd be prospecting their own neighbourhood planets, or at least some planet a little closer to them? Which brings up this question.

Who's piloting all these UFOs and are they biological, or robotic, or both?

What if an alien civilization on a distant planet, millions or even billions of years more advanced than man developed bio-robots, which over time developed into machines with their own intelligence and a soul. What if they were programmed to recreate themselves because the alien race that developed them were dying out and actually had died out, billions of years earlier? The robots lived on, re-creating themselves over and over until, they became the perfect machine and I might also add, in the likeness of their creator, but still looking alien to us.

They could travel the skies forever, piloting the UFOs, needing only a little oil now and then. They'd need no food, hell, they might not need air, or water, with batteries that would put lithium to shame. No need for a UFO craft, maybe they use a jet pack, hanging in space until it arrived at a new destination, Earth. 

Possibly, hanging on for millions and millions of years, travelling a zillion miles and depending on when their alien civilization died, they could have left their planet millions of years ago, and will be arriving here soon. Possibly not just one but many. Could you imagine millions coming all at once?

Of course, I'm just fantasizing, it's all Science Fiction even though it does make more sense, that aliens will be bio-robots or just plain robots, eh?