There are a great many reasons why airlines choose to lease aircraft. Aircraft leasing is a growing industry.
To put it in perspective, near enough 50% of British Airways fleet is leased. So why would a self-proclaimed world-class former flag carrier choose to do this?
The reasons are many and complex but include : tax law, greater ability to fix costs, lower cost financing, lower gearing ratios, ability to show a greater return on capital invested, very favourable purchasing terms able to be achieved by the leasing companies etc etc.
It works, and it works very well.