The nice thing about vegetables having fewer Calories is you can eat more vegetables and get 10 times the amount of nutrition than you would with an equal Caloric amount of your favorite cut of steak. In fact I consider most meat pretty devoid of nutrition. Other than a few excessive amounts of stuff you don't need a lot of at any one sitting. Of course there are plenty of vegetable that are lacking in nutrition too. So you have to know what is and what is not packed with lots of various nutrition.
Some vegans/vegetarians do make the mistake of eating too much pasta and stuff that is mostly just starch, like potatoes. But if you've gotten a decent amount of nutritious food, then there is no overwhelmingly bad reason not to use nutritionally empty carbs to replenish what you lost from hard work or exercise.
Fats, carb's and proteins in themselves are not nutrition. Just fuel and building blocks.
No, I'm not vegan...... I just don't eat a lot of meat anymore. Particularly not beef... though when I'm at a place known for exceptional prime rib, I do partake.