Executives build the company that the investors want them to build.

By far, the biggest costs of running an airline are the planes and the fuel. But the investors don't want to bet on the value of physical planes, nor do they want to bet on the price of oil. If they wanted to place those bets, they'd just invest in Boeing or Exxon. Instead, they usually want to bet that one airline will perform better than her competitors over the next year or so.

So, airline executives lease their fleets and buy tons of oil futures. This gives them a better shot of hitting their targets even if the price of oil skyrockets, it makes their fleet easier to scale up or down according to demand, and it makes their stock more attractive to investors who want more predictable performance.