70 % of muscle is water. Less than 30 % is protein. 10 kg (22 pounds) of muscle contain less than 3000 g protein. It takes several months to build that much muscle.
That is bodybuilders need 3000g extra protein per few months which is not that big.
Physical Fitness Stack Exchange is a question and answer site for physical fitness professionals, athletes, trainers, and those providing health-related needs. It only takes a minute to sign up.Sign up to join this community
There are already some answers that address this, basically tissue is being torn down and needs to be repaired. But it goes further: there are simply a pile of benefits to upping protein (2012 study):
Protein seems to play an important role in the emergence of [feeling full]. Long-term ingestion of a high-protein diet not only decreases food intake but also lowers animals’ body weight and reduces body [fat deposits] in animals and humans.
Just at a baseline (no strength training involved), a 170lb man would need 61.2 grams of protein according to the US RDA. So bumping that up to the generally accepted "1 gram of protein per pound of bodyweight" is a 110 gram bump from the baseline.
Beyond the gram-for-gram measurements, there's a lot going on about availability (2005 review):
The response of muscle protein metabolism to a resistance exercise bout lasts for 24-48 hours; thus, the interaction between protein metabolism and any meals consumed in this period will determine the impact of the diet on muscle hypertrophy. Amino acid availability is an important regulator of muscle protein metabolism.
Boiled down, there are three reasons for the protein recommendations:
1) Muscle Repair. You are destroying muscle tissue when you do resistance exercises, and your body needs building materials (protein) repair that damage:
Resistance training leads to trauma or injury of the cellular proteins in muscle. This prompts cell-signaling messages to activate satellite cells to begin a cascade of events leading to muscle repair and growth.
2) Carbohydrates. There is a lot going on with carbohydrates and although your body needs them, they tend to make up too much of a western diet and need interfere with muscle growth via insulin response, and promote fat storage. If you have a 2,500 calorie full of carbs and you increase your protein, chances are you'll be opting for chicken instead of bread.
3) Availability. Muscle repair can take dozens of hours to complete, during which the easy access to amino acids promotes tissue repair and regeneration.