[Nasrudith]

I am no mining expert, but wasn't ancient copper mining also pretty much "on easy mode" due to more surface deposits of higher concentration near native copper and thus less processing required? The later engineers didn't have the luxury of persistently abundant resources that let them just leave valuable ores as slag.

Meanwhile just digging deeper tends to lead at very least to more incidental runoff pollution spreading heavy metal infused mud. Let alone spread of pollutants from other more advanced practices like chemical processing extracting copper and pollutants from slag and undoing the containment effect.