The production possibility curve (PPC) is drawn as a concave shape bowing outward from the origin primarily because of

The PPC is concave (bowed outward) because resources are not perfectly adaptable between uses. As more of one good is produced, increasingly less suitable resources must be transferred from the other good, causing the opportunity cost of each additional unit to rise — this is the law of increasing opportunity cost. Diminishing marginal utility applies to consumption, not production possibility.