Because there can’t be an out-of-court settlement of the lawsuit against Amphastar unless both NVS and MNTA agree to the settlement terms.
Why? Certainly there are plenty of licensing agreements where the originator licenses everything to the licensee in exchange for a royalty. Which is a form of pre-specifying the patent suit split.
That said, I agree that there are a lot of potential moving parts which would make it difficult to fully pre-specify a patent settlement split - but I could see a statement of intent to be enforced via arbitration (if necessary).