There were no flat panels. That was a sales pitch. In reality, a slight curve to a panel adds so much strength that it can be made considerably lighter/cheaper than a flat panel of sufficient strength. Flat panels also tend to dent inwards very easily, which is not a good look for a car. Then there is the issue of thermal expansion. A pre-bent bit of metal will expand in a predictable way. A dead-flat panel could buckle in multiple unexpected ways.

I had a jeep wrangler once. The window was "flat" but not if you looked at its reflection. There was an almost imperceptible but complex curve, no doubt for strength.