If nothing else, because it would be prohibitively expensive. Globally, something like 70 million barrels of oil are produced per day. The total value of all barrels produced in a year varies depending on the price of oil, but at a highish but realistic $100bbl, you're talking about two and a half trillion US dollars per year. If you were to reduce the supply by introducing a 'buyer' (read: subsidy to defer production) for some large percentage of those barrels, then the price would go even higher; this project would probably cost more than the entire global military budget combined, with no immediate practical or economic benefits.