But the licensing didn't change between gcc 12 and gcc 16, so that doesn't seem like a good explanation?

It is not a good explanation because that is not the reason. That reason is for Mac OS. I don't know the details, but BSDs normally modify the GCC version they use. IIRC OpenBSD used to use GCC 2.95 for years. Porting NetBSD base to compile with a newer GCC version is not as simple as a version bump. My guess is that they haven't prioritized that work yet.

Patches welcome!