Why are oligopolies able to earn both short-run economic profits and long-run economic profits, while price taking firms like perfect competitors can only earn short-run economic profits?

Why are oligopolies able to earn both short-run economic profits and long-run economic profits, while price taking firms like perfect competitors can only earn short-run economic profits? Discuss the characteristics of perfect competition and imperfect competition (monopolistic competition, oligopoly, and monopoly).