The thing is, that if there are expansionist territory claiming ETs in our galaxy - it is very very unlikely they evolve at the exact moment of geological time as us.

Humans have had technology for only a few centuries. And space technology for less than a century. It took us billions of years to evolve on Earth.

ETs also would live in a universe that is many billions of years old, and whatever their origins, there is nothing special about this particular moment of time. We haven't changed the habitability of the rest of the galaxy (not yet anyway) by evolving on Earth,

So, they could have evolved any time in the last many billions of years. It doesn't matter if they evolve on the surface of a neutron star, or planets around red dwarfs, or in subsurface oceans in tidally heated moons orbiting rogue planets or brown dwarfs, or whatever unusual habitat you posit - all those habitats have been around in one place or another in the galaxy for billions of years. 

Then, at a moderate average speed of expansion of a tenth of light speed (assuming advanced technology surely that is not fast for them), it's only a few million years for an expansionist ET to colonize an entire galaxy, and it took billions of years for our civilization to evolve.

So if our planets were of interest to them for mining - they would already be here mining them and would have been here for millions of years. We'd see their mines on the planets already.

And our solar system is pristine as far as we can tell. There are no ET tracks or mines on the Moon or Mars or anywhere else we have looked at close up. On Mars especially we can photograph any of the planet at well sub meter scale.

If they have been here for millions of years then they are obviously non interventionist.

And obviously also there haven't been waves of colonization that have passed through our solar system and then the ETs responsible gone extinct. Because we'd see their tracks alsos.

I think it is much more likely that they are not here, and that any ETs in our galaxy are non expansionist.

As to why that is, I think myself that that's because any sensible ET will think ahead and look at the consequences of unrestricted colonizing of a galaxy. It would involve most of their population ending up well beyond any possibility of communication, and that would make it impossible to deal with the ones that through foolishness or intent end up trashing the galaxy. And what's more the fastest expanding most reckless members of their civilization would be the ones that would end up inhabiting all the stars in the galaxy.

I think any sensible ET will look at that scenario and decide they need to find a different future path. Setting off waves of colonization in the galaxy is at least as dangerous as setting of waves of self replicating robots - which is what many deal with as the nightmare scenario. But robots can be programmed and controlled. Humans or ETs can't - and they can also create the self replicating robots and program as they like, perhaps forgetting the reasons for safeguards to moderate their self replicating capabilities of their more sensible parent civilization.

There may be ways through this. But if so - if there is some ancient galaxy spanning civilization - it clearly is not one that claims territory throughout the galaxy and then mines all the planets. Because if it did that, they would already have mined our planets millions of years ago, probably billions of years ago.

It makes for fun science fiction. I like Star Trek, Star Wars, Doctor Who etc. But science fiction is not future or present day history. And often gets things wrong. Most science fiction either doesn't address this at all - or uses implausible ideas, such as the idea from Star Trek of the ancient hominids that somehow "programmed" ancient single cell lifeforms on many planets throughout the galaxy so that they all simultaneously evolve human like lifeforms with technology within a few centuries of each other billions of years into the future. That's a fun science fiction idea, but not very plausible :).

See also: Self Replicating Robots - Safer For Galaxy (and Earth) Than Human Colonists - Is This Why ETs Didn't Colonize Earth?

Why Didn't ETs, Or Self Replicating Machines, Colonize Our Solar System Millions Of Years Ago?