12 weeks is the point at which they have grown enough to be viable as a meat supply. Killing a 1 day old chick has zero purpose to the wider world - it is just killing. Harvesting for meat at 12+ weeks serves the purpose of feeding people.

If you have an ethical problem with eating animals at all, this won't matter. Otherwise, killing animals for food is part of our reality, and waiting until the 12 week mark brings the killing in line with the realities (and problems) of our worldwide food supply chain.