system
Given the following post, respond as if you're commenting on Reddit:
human
[Stargate] Where do the Ancients get the materials for all there constructions?

Lets brake it down a little:

While they were technologically semi-advance (incapable of true FTL):

Unknown number of Seed Ships (probably in the thousands) hundreds of meters long, made out of durable dense materials so probably saying million of tons heavy wouldn't be a big overstatement. All of which at least equipped with a certain number of Stargate to begin with.

Destiny, 2 to 3 times bigger then the Seed Ships.

Unknown number of stone-metal constructions on planets serving as bases and cities.

Once they got more advance:

Unknown numbers of City Ships (comparable or bigger then Destiny) they were build while still in Milky Way  (maybe not thousand but at least near the hundreds again).

A Stargate system spanning along a whole galaxy

And again updated City's build out of stone-metal around the galaxy and again an unknown number of outposts.


So my questions becomes the Ancient where very very active in there days, building settlements, building and sending ships out there yet we don't have any evidence of them doing any mining or Stargate personal mentioning finding planets with Stargate's which are mined for all they are worth. 

Even if they somehow utilise whole planets the infrastructure needed for something like that must be huge and can't simply disappear. Furthermore Earth was there supposed base and adopted home yet there is no sign of any harvest done on it ether.
gpt
ZPMs aren't generators. They are literally creating energy from nothing (well, technically from accelerated entropy of a artificial pocket universe). We have also seen Ancient technology capable of compiling mater from energy. 

Simply solution.. once they developed ZPM tech, which seems to have been before their arrival on earth, their need for resource extraction ceases.