Because people tend to stir in pots, leaving small scratches in the coating. You also use a LOT more heat under a pot (especially using propane), than putting stuf in an oven, letting things burn to the surface.
Pots are used where there's a risk of boiling over... Lot of surface/ridges for cleaning up burnt (since flames are licking up the side) milk/cream. Ever tried to clean that on an "easy clean" surface...
And don't you just love how:
Video: conventional uses 40% more energy (reduction of ~30%)
Article: uses 40% less energy