Veganism is not advisable because it's a diet naturally high in carbs
"Vegan food" to carnists consists of taking their normal diet (of meat, cheese, and bread exclusively) and subtracting out the meat and cheese to be just left with bread. So of course, "vegan" is synonymous with "high-carb" because the only vegan things they ingest are high-carb junk foods like mountain dew and potato chips. This isn't a failure of veganism or representative of a well-planned vegan diet.
Perhaps if there were more low-carb vegan junk foods (like recent releases of vegan beef jerky or high-fat low-carb vegan cheeses) then this might be corrected, but this would require more carnists to sign on to veganism.
Carbs sound terrible but there are three macronutrient sources of calories: carbs, fats, and proteins. Certainly, if vegans were mentally associated with a high-fat diet people would have this same knee-jerk reaction that this is somehow unhealthy.
But then what is left if one is to say that carbs are blanket unhealthy and carbs are blanket unhealthy? Just eating 100% protein?
There are plenty of resources to eat low carb as a vegan, and plenty of online communities dedicated to the pursuit. Just like low-carb carnist diets, these focus on incorporating other foods into the diet including vegan foods, like red lentil pasta and other nutrient-dense foods that carnists will happily integrate into a keto diet.
Meat-containing or not, these diets take work. Carnists are happy to try new spice rubs on their meat or introduce new vegetable pairings in an attempt to eat low-carb, but when it comes to vegan food they aren't willing to extend that same effort, so they throw their hands up and prematurely claim defeat. That's not a good argument for hurting animals, though.