this is driven by the fact that airlines don't consistently have taxable income. If you have net operating losses, which airlines often do, you can't fully take advantage of the tax shield provided by depreciation. Airplanes are big capital assets, hence they provide a large tax shield. That's why you often see financial entities (e.g. banks, insurance companies) who have regular taxable income have leasing arms.

Edit: I stand corrected... Although I still wouldn't be surprised if the execs golf together. :)