Why can't I just go for a straight high-protein diet?

The argument is that since muscles are made of protein, so eating a high protein diet helps build muscle. But it ain't necessarily so.

Without sufficient carbohydrate intake for energy, protein is used for energy production instead. Although strength athletes and body builders do need more protein than a sedentary person, studies have shown that even for these people, there's little benefit consuming more than 2.5 grams of protein per kilo of body weight per day. That's around 200 grams per day for a 70kg athlete.

Remember this is the absolute maximum recommended protein intake; the British government's Committee on Medical Aspects of Food and Nutrition Policy (COMA) guidelines suggest that around 55 grams of protein per day is sufficient for a male adult.

Moreover, very high protein diets place extra demand on the kidneys, which have to dispose of the unwanted nitrogen. The consensus is that plentiful, but not excessive protein intake is the best approach for lean muscle gain and this should also be combined with high carbohydrate nutrition as explained here.