It's not the amount per liter that matters, it's the proportion of proteins vs sugars vs fats
Milk powder (fat or skim) has ~53% sugars and 35% protein, by weight. Good protein powers are usually 70+% protein by weight. So 2 scoops of milk powder is the same amount of protein as 1 scoop of protein powder, and a hell of a lot cheaper. But you have to be lactose tolerant and also balance those sugars out in your diet. It's much easier to do with the concentrate. Also, using milk to bump up the protein levels to the point where bodybuilders need it means you're absorbing tons more sodium compared to the equivalent amount of protein powder.
Why would you mix protein powder in milk, though? Mill is already nature's protein drink. Just have a second glass of milk if you need more protein.
Protein powder is whey concentrate. Whey concentrate is much faster to absorb, so they use it after workouts. The main scientific reason (though a lot of people use it without understanding it) is not "bulk up quickly" but to prevent the
catabolic breakdown of muscles which happens in the aftermath of heavy workouts where your muscle glycogen stores are depleted: by drinking a mix of rapid-absorbing sugars (best: glucose) and proteins it minimizes the time that your muscles are in a catabolic state, thus keeping up the muscles you're trying to build. Knowing what it's for, you can realize you don't actually need that much whey powder or that often. Only when you're working out. Use other cheaper and slow-absorbing proteins at other times. e.g. casein (from milk, or better cottage cheese) is a good choice before bedtime, because it is slow absorbing and avoids your muscles depleting overnight due to catabolic effects.
So that's why whey powder exists and what the real purpose of it is for. Whey is the fastest absorbing protein available. Given that, why put milk in
it is the question. Because (1) it tastes better. (2) they're a different type of protein with very different metabolism. Milk = casein (slow release), prot.powder = whey (fast release). The fast-release protein is to restore muscle nutrition more quickly after workouts. The slow-release protein is to maintain long-term muscle development. Mixing the two types in post-workout drinks seems to be the most effective choice according to a number of studies.