Burgers look absolutely nothing like a dead animal. Carnists have already done the work to remove the imagery of the act of violence from eating meat. Most vegans wouldn't eat e.g. a vegan rotisserie chicken because that actually does look like a murdered animal.
Also, you can't be vegan for environmental reasons. Veganism is explicitly about ethics
So as not to muddy the vegan movement. Veganism is explicitly about ethics, and stretches beyond a diet and into anything else that involves animal products