I think getting some quality protein, good carbs, plus vegetables and/or fruits with most meals will constitute a healthy diet. I would try to avoid fried foods and sugar, but that stuff is okay in moderation. Is it an exact science as to what each person should eat for every meal? No, but that's why people should find out what works for them.
Think about it like this: boys enter their prime growth years in their teens. That is when their bodies require more nutrition than at any time in their lives if they're going to reach their peak height. And teen boys can eat a LOT of food. They seem to always be hungry and that makes sense because they experience rapid bone growth along with all the muscle tissue that attaches to those bones. I've noticed that the taller boys are more likely to consume lots of whole foods - they crave them - such as meat, milk, cheese, eggs. And while some of them do take vitamins, many of them don't and it doesn't seem to affect their height or overall health.
So why, as an adult, would a weightlifter need anything other than quality whole foods to provide the nutrition that he needs? He's not getting any taller, his bones have stopped growing, his tendons & ligaments aren't getting bigger, and all he's trying to do is get stronger and increase muscle mass. It makes sense that good food would provide all the nutrition that he needs.
It's something to do with the way supplemented amino acids are processed in the body as opposed to how naturally derived amino acids in food are used by the body.