Why markets can only supply private goods efficiently?

Private good is a product and/or service produced by a private business and purchased to increase the utility and/or productivity of the buyer. The majority of the goods and services consumed in a market economy are private goods, and their prices are determined by the market forces of supply and demand. Private goods are both excludable and rivalrous, where excludability means that producers can prevent some people from consuming the good or service based on their ability or willingness to pay and rivalrous indicates that one person's use of a product reduces the amount available for use by another. In practice, private goods exist along a continuum of excludability and rivalry and can even show only one of these traits.

Answerdiscriminant validity;