Why is the production possibilities frontier usually a bow-shaped curve instead of a straight line?

Question

Quizplus · Accepted Answer

The production possibilities frontier is usually a bow-shaped curve because resources are not perfectly adaptable to the production of all goods. In other words, as more resources are devoted to the production of one good, the opportunity cost of producing another good increases, leading to diminishing returns and creating a bow-shaped curve.

Why Is the Production Possibilities Frontier Usually a Bow-Shaped Curve