I believe I just did? My argument is that despite morals not coming from some magical entity, they have an origin in humanities success in society, and are therefore still important. For something to be immoral doesn’t merely mean an entity says it is bad, it means that thing goes against principles which benefit our societies. Murder is immoral, not because an entity decided that, but rather because societies which accepted murder were far less successful than societies which did not.
For veganism, the environmental mortality is clear. Besides that I suspect the reason we tend to see unnecessary animal abuse as immortal is because kinder humans tend to be better for society, and kinder humans also tend to be kinder to animals, not just humans.