Because oil prices won't stay that low forever, in fact Saudi-Arabia and other OPEC members lowered the production quotas to get higher oil prices. The budgets of many countries with natural resources expect a certain price floor for oil - this especially hits Saudi-Arabia as they don't have much economy except oil and a tiny bit of Mecca tourism, but also Russia which has been hit hard by the drop in oil price and which doesn't have any kind of industry that's comparable to oil in terms of exports.