But isn't it just an electrical discharge between thunderclouds and the ground?
In a sense, but the big problem is that to get a spark, air needs to break down. It needs to stop being an insulator and start being a conductor. We commonly experience this if you touch a doorknob and you get a spark between your finger and the doorknob. What happens is the charges get concentrated into your fingertip and you get a big electric field. Then, as your finger approaches, the conventional breakdown field is reached, which is about 3 million volts per metre – and then air sparks.
The problem is if you look up inside thunderclouds, the breakdown field that you need to make a spark is never found. People have been launching balloons for decades, they've been flying airplanes, they've been launching rockets...but the fields they record are not even close to this strength.