[steve-b]

The simplest answer is that, to get to another star system you have to adapt to living in an "interstellar ark" -- and once you've done so it's easier to just take what you need from asteroids and comets than to re-adapt to living on planets.

For all we know, there could be thousands of old mining traces in our own Belt.

[RightWhale]

there could be thousands of old mining traces in our own Belt

Seeing as how we don't have much info on what happened right here on earth even 10,000 years ago, and we have just begun to look around the other planets and asteroids in the solar system, the possibility that others have come and gone many times cannot be denied. Just because we don't see them now doesn't mean they never passed through gobbling up this and that on the way.

[#3Fan]

For all we know, there could be thousands of old mining traces in our own Belt.

Like the perfectly square tunnels of South America.