>>538927171
Because even if we did call him in, Rerir can't be killed. He's transcended death. Flins literally explains this when you're going over the details of your mutual encounters with him.
So even if we DID call in Neuv, it's not a viable solution since he may be able to deal with 10% Rerir, but might struggle with him at 20% or 30% or higher, and if the other Sinners are anything to go by, we know for a fact that Neuvillette isn't strong enough to defeat one at 100% power. The solution will be need something that doesn't involve raw power, so why even waste the energy?