My Christmas Cockerels are now 28 days old and in three more weeks will change their diet to my special one of non ACS growers, grapes. apples, milk, wheat, maize, and free range grass. This year, I am a bit later than usual and so they will kill out at around 12lb to 16lb.
The odd one or two will go off their legs and so, minus a leg or a wing, will go in my freezer for eating later in the year.
As for Tesco et al. They are organisations comprised of managers who act in accordance with normal patterns of commercial corporate social behaviour. In short, whatever they can get away with commensurate with their corporate goals. If their grandmothers could command a sufficiently good margin, they would sell them.