I understand it - but then I have also flown high performance gliders. And I have often thought the same - although I understood negative flaps to be more about dumping unwanted lift and also drag. But as some of the team who designed the modern Airbus aircraft had also designed gliders (the A320 has an aerofoil section based on a common glider section) I would be surprised if they hadn't considered it. The fact they don't have them probably means they weren't deemed to be workable.