Because factory farming _is_ the most environmentally sustainable form of animal agriculture. Environmental sustainability and animal welfare are in competition with each other, to give to one you have to take from the other, so the obvious choice is to simply reduce animal agriculture.
Not enough to change their diet though.
> why not push sustainable agriculture instead
Because factory farming _is_ the most environmentally sustainable form of animal agriculture. Environmental sustainability and animal welfare are in competition with each other, to give to one you have to take from the other, so the obvious choice is to simply reduce animal agriculture.