Why is a production possibilities frontier typically drawn as a curve, rather than a straight line?

PPC shows combinations of two goods, that an economy can produce with given fixed resources & constant technology.

As resources given are fixed, more units of a good can be produced by sacrificing units of another good only. So, the goods are inversely related & the curve is downward sloping.

The amount of a good sacrificed to gain an additional unit of other good is called Marginal Opportunity Cost.

This MOC - ie a good sacrificed to gain additional unit of other goods, keeps on falling as successive units of latter good are gained. It is because resources are assumed to be not equally efficient in production of both goods, their shift from a good to other implies increasing sacrifices.

So, the slope of PPC ie MOC is increasing & hence the curve is concave to origin.