You could think this in terms of an oligopoly. As an imperfect market, firms get more profits than possible in a competitive market because decisions around levels of production influence prices. All members of the OPEC have an incentive to reduce production as a whole (to raise prices), and an incentive to increase production individually (to earn more profits at those higher prices).
You can also consider political issues like production outside the cartel: they don't hold a monopoly and the higher the OPEC raises prices, firms with costlier ways of extracting oil (like fracking in the US) can turn a profit and compete with them, which naturally reduces profits (and value).
While the IPO may be a possible explanation, you can think of it in terms of oligopolies and still get a reasonable answer.