Why do prices fail to represent the opportunity cost of resources when externalities exist?

Prices fail to represent the opportunity cost of resources in the presence of externalities because they do not account for the full social costs or benefits associated with a good or service. For example, when a factory pollutes the air, the negative impact on public health and the environment is not reflected in the price of its products. As a result, resources may be over-allocated to the production of goods with negative externalities, leading to inefficient outcomes that do not consider the true cost of resource use. This misalignment distorts market signals and can result in suboptimal resource allocation.