One reason why a lot of veggies and vegans may not eat healthily is the difficulty of getting healthy vegan/veggie food when eating out, e.g. at work, school, restaurants, other people's homes, etc., and the poor nutritional quality of veggie convenience foods. I think it's very much a cultural thing. Many is the time I have found that there is absolutely nothing for me to eat in a pub or restaurant, and once even at an event where I had been promised that I would be catered for, other than chips ('fries' in US-speak) and/or a bit of salad. People who are veggie or vegan also need to think 'outside the box' more. Gluten-free/low-grain/low-sugar adds another layer of complication and demands a bit of inventiveness. But I don't find it difficult when at home, and there are some eateries that cater well for me now, even in the relatively-unenlightened Duchy of Cornwall. But it should still be borne in mind that most scientific studies show that veggies and vegans are at least as healthy as omnivores.