For a lot of people veganism/vegetarianism is all about feeling morally superior to others. For Grug it's definitely a large part of the appeal. We know he doesn't give a shit about animals, otherwise he'd have cleaned his Guinea Pig's cage and trimmed it's fucking nails. He also wouldn't keep his dogs penned in a kennel with garbage. He most certainly wouldn't have murdered a fucking tortoise. So he can't claim he's doing it out of love of animals, Grug doesn't care about animals any more than he cares about anything else that isn't him.
And considering the damage he's done to wetlands and his constant burning of cardboard in his backyard, he can't claim that he's vegetarian for environmental reasons. environmentally conscious people don't cut down a living tree and then shove its remains into a wetland. Even if he had gotten prior permission to cut down that tree, had he been environmentally responsible he would have called a service to come haul that thing away so that it could be mulched down.
Dumb fuck can't even claim that he's doing it for health reasons since we've seen time and again absolute proof that he eats nothing but pre-packaged, processed crap and fast food. If this nigga went vegetarian for health reasons he and Lainey would be buying armfuls of fresh vegetables and grains to make healthy homemade meals.
Plain and simple, Guug only cares about vegetarianism as much as he can use it as a moral weapon to beat other people over the head.
"What's that? You think I'M a piece of shit? Well, uhhh, YOU MURDER ANIMALS AND I DON'T! You're the real bad guy here!"
Honestly, I think it stems from a very very childlike mentality where doing perceived good thing makes you good. Vegetarianism is good, and good people are vegetarians, therefore Grug is good because he's a vegetarian.
It's an insanely simplistic and naive mindset that completely ignores the fact that Adolph fucking Hitler was a vegetarian. What's more, he was one of those obnoxious and hypocritical vegetarians who, while still having a soft spot for certain meat dishes he couldn't resist and continued eating against his doctor's advice, loved to annoy his dinner guests by preaching at them about how badly the animals they eat suffer. Not shitting you, Hitler was one of THOSE vegetarians. Just like Grug.