Researchers have linked dietary data from over 55,000 individuals with data on the environmental impacts of the foods they eat. The team, from the Livestock, Environment and People (LEAP) project at the University of Oxford, found that the dietary impacts of vegans were around a third of those of high meat eaters. They also saw a 30% difference between high- and low-meat diets for most of the measures of environmental harm.
From what I’ve seen its often just genuine ignorance. Thinking plants are entirely carbs, needing protein for muscle so of course you only get protein from animal muscle, and that old myth of incomplete proteins. Ill be honest I had the same assumption that something like a potato is entirely carbs