Did you miss the part where the Night King can just RAISE THE DEAD?

Conducting hit and run tactics, the Dragons might reduce the numbers by a few. But many of the bodies would survive intact enough to simply be raised back up. It would take probably hundreds of sorties to make a sizeable dent in the army of the dead, and if you recall, Dany mentioned that it was too cold for them, and they were having trouble getting enough to eat.

Attacking 1% of a massive zombie mob, only to have 80% of them get back up afterwards, over and over again would tire the dragons out and make them unable to help in the coming battle. Not to mention putting Dany and Jon and the Dragons at risk of an Ambush.