@Mfauli - Maybe there are and they all failed? (EDIT: Y beat me to it)
The point I am making is the same way we are making assumptions, you and Darth are also making assumptions. As long as it makes sense with the rules of the world and can be explained and/or inferred with information we do know, that isn't bad writing or characters being stupid. If you wish it went another direction, then that is absolutely valid.
@Darth - Many countries have stopped producing nukes because the ones they have are enough to destroy the world many times over. There is simply no need for more. If Frieren can teach Fern all the destructive spells she will ever need, a practical daily-use spell is more valuable at the moment.