Why do some people think vegan/vegetarian diets require so much planning...?

Some users have posted on this section and said things like "if a vegetarian plans out their meals well and takes supplements they can be healthy". Considering I myself am a vegetarian and I don't preplan my meals for the most part, unless I want to whip up something yummy and need to shop for the ingredients, and also considering I don't take supplements and am completely healthy, why do some users on this section continue to post lies? Wouldn't the people who know the most about how "difficult" the vegetarian and vegan diet is be, I don't know, vegetarians and vegans?