Because I am a young (and small) mammal that is still growing and hoping to put on as much muscle as possible. :)

I'm quite literally who milk was made for.

Your point about species is valid though and I can't say that if given an ample supply of human milk, I would be inclined to consume it.

Sure milk will help to build them! Vegetable will, too (see gorillas, the biggest and strongest humanoids) or soy for a boost. I don’t think being young add anything, a 40yo could also want to build muscle. You mention that you’re still growing, milk can help here too but also an exceptions: not a single mammal drink milk after babyhood.