The reason that the production possibilities frontier is usually a bow-shaped curve instead of a straight line is that:

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The production possibilities frontier is usually a bow-shaped curve because resources are not perfectly adaptable to the production of all goods. This means that as more of one good is produced, the opportunity cost of producing that good increases, eventually leading to diminishing marginal returns and a decline in the production of the other good. A straight line would imply that resources can be easily shifted between the production of any two goods at a constant opportunity cost, but this is rarely the case in real-world economies.

The Reason That the Production Possibilities Frontier Is Usually a Bow-Shaped