I've been a vegetarian for nearly 40 years, vegan for 25 or so, and a competitive athlete for a solid portion of that time.
Human beings are omnivores; we have tearing teeth in the front to eat flesh, and grinding teeth in the back, to eat grains and vegetables. Our digestive mechanism is not as brief as a pure carnivore's, nor as convoluted as a strict herbivore. But...we also have outsized brains, and hands that can manipulate our environment. We don't live in a state of nature, and we can make choices that other animals can't. We have the tools to acquire complete proteins, a full complement of B vitamins, and appropriate amino profile--without meat, and without much effort.
People choose vegetarianism for different reasons. Some do it for health, some aesthetics, some for political, religious, spiritual reasons. Some embrace it for a short time, some for life. Not all even eat a particularly healthy diet, especially starters and dabblers. Most who stick with it evolve a personalized approach that works for them. Even people who give it up emerge with a greater awareness of the consequences of what the put in their mouths--for themselves, and for their neighbors.