This is one of the least wasteful things they do. The cuttings are recycled, and aluminum is one of the few materials that is more energy efficient to recycle than to mine/manufacture from scratch.
They also do CNCing pretty efficiently. The same piece of aluminum that makes an iMac screen bezel makes two wired Mac keyboards.
You know exactly why: you don’t get the quality that Apple wants any other way. Supposedly they reuse the shavings, so it might not actually be as bad as you think.
And when the machine is decommissioned, the aluminum from the chassis is easily recyclable...