carbohydrates and lipids as a source of energy? why?
First, it is a matter of the energy that food can provide. Here fat wins clearly (from the Wikipedia article on Food Energy):


Fat: 37 kJ/g
Ethanol (drinking alcohol): 29 kJ/g
Proteins: 17 kJ/g
Carbohydrates: 17 kJ/g
Organic acids: 13 kJ/g
Polyols (sugar alcohols, sweeteners): 10 kJ/g
Fiber: 8 kJ/g


Proteins and Carbohydrates have approximately the same energy content. But the advantage of carbohydrates (and especially glucose) is that is can be taken up by the body and the cells pretty fast, while proteins need to be digested and broken up, before they can metabolized further. Also plants make primarily sugars (or derivated polymers like starch), so carbohydrates are readily available in the food.
Fat and sugars are chosen for nutritional reasons, since they have a higher energy content and are available for it.

Structural components are build by proteins, since the 20 amino acids allow a much greater diversity in the protein chains than it would be possible for fats or carbohydrates. The building blocks are relatively small and simple but they all have the same basic structure (carbogroup, aminogroup and the sidechain) which allows linking via the functional group and the sidechain influencing their behavior (hydrophobic, hydrophilic, charged, etc.).