[BACKCHECK!!!]

Just so you know where I'm coming from on this, I'm a medical doctor, and I have a physics degree. So I know a few things about biology and energy.

For starters, muscle repair is an energy-intensive activity. When you shred a muscle during a work-out, you are not just burning calories by doing the workout. You are also burning calories during the repair process. You are taking digested simple amino acids, and assembling them into functioning sarcomeres and myofibrils. This involes a vast number of biological processes. Taking something simple, and making them into something complicated, requires moving a ton of entropy, all of which takes energy. Don't tell me how much protein is in a muscle, tell me how much energy it takes to make it. Building muscle takes more than just the protein that's being used up. It takes energy. Loads of it.

You're basically saying 'All it costs to build a car from scratch is a couple hundred dollars, because you've only got to pay for a few hundred kg of steel.'
We know what it's made of, but it takes a lot more than that to make it.

And that's just for starters.

There's also the whole endochrine side of things, which I'd say is even more important than just the amount of macronutients going in and out. I'd hardly call myself an expert on that subject (I wouldn't be surprised if there's fitness geeks on this forum that know more about the specifics than me). But I can assure you there's a whole heck of a lot more to it than "If you want to gain 10lbs of muscle just eat an extra 2.2lbs of protein."

The internet took to the idea that you need to eat lots to gain a lot of muscle muscle mass because it is correct.