They eat those meals to support their exercise regime so that they can get bigger.
Why would they want to lose weight they are 'body building' after all !
Then, afterwards, they eat protein to provide the "building blocks" for new muscle.
In the evening, when they finish working out, they stop eating carbs, because they won't burn them off.
This is a simplified explanation.
If you want to see how much carbs are needed for energy, Google how much pasta and eggs a Tour de France cyclist eats for breakfast. You will not believe it.