Because each time you look, all this dross is burnt.

The laborer, looking into it at evening, purifies his thoughts of the dross and earthiness which they have accumulated during the day.

He sends down water from the sky whereat the valleys are flooded to the extent of their capacity, and the flood carries along a swelling scum. And from what they smelt in the fire for the purpose of making ornaments or wares, there arises a similar scum. That is how Allah compares truth and falsehood. As for the scum, it leaves as dross, and that which profits the people stays in the earth. That is how Allah draws comparisons.

Allah sends down water from the heavens and the river beds flow, each according to its measure, and the torrent carries along a swelling scum. In like manner, from that metal which they smelt in the fire to make ornaments and utensils, there arises scum like it. Thus does Allah depict truth and falsehood. As for the scum, it passes away as dross but that which benefits mankind abides on the earth. Thus does Allah explain (the truth) through examples.