Milling is great, but not very mass-efficient. Resisting the cutting forces involved in milling anything harder than light foam, requires a lot more structure than simply moving a deposition print head. Machinists use mass as a shorthand to talk about a machine's rigidity, with anything less than half a ton being basically a toy.

Remelting waste is possible, but this sort of high quality manufacturing is really hard, even on earth with gravity to keep material in a mold and hardly any mass or energy constraints. This, and the chip collection system add additional mass, have to be developed way before being put into orbit, and have to work extremely reliably. Even a tiny amount of swarf floating around the interior of a spacecraft could be disastrous.

We aren't focusing only on 3d printing, we're focusing first on 3d printing, because it can work today, with technology we have, on a low mass budget, and without jeopardizing the primary objectives of whatever mission includes it.