Final answer:
Perfect monopolies are rare mainly because of government intervention, which includes regulation of natural monopolies and enforcement of antitrust laws to encourage competition.
Step-by-step explanation:
One reason why perfect monopolies rarely occur in practice is because of government intervention. In a perfectly competitive market, companies are free to enter and exit, but monopolies are characterized by the absence of competition and barriers to entry. These barriers could be economies of scale, control of resources, or intellectual property laws such as patents and copyrights. However, natural monopolies occur in markets where the most efficient scale of production is so large that it costs less for one firm to supply the market than for multiple firms to do so. These are often regulated by the government to prevent the high pricing and inefficiency normally associated with monopolies. Government interventions such as antitrust laws, price controls, and direct government provision prevent the formation of undue monopolies and ensure fair competition.