One could, but one probably doesn't want to have separate types for TCP-over-IPv4 vs TCP-over-IPv6 for example, even if they accept/produce different forms of addresses. That'd force a lot of code bloat with monomorphization.

So now one is making one's own enumeration which is different than the OS one and mapping between them, which can get into a mess of all the various protocols Linux and other OSs support, and I'm not sure it's solving a major problem. Opinions vary, but I prefer to use complex types sparingly.

I think there are likely a bunch of other cases where it's useful to choose these values more dynamically too. Networking gets weird!

It's precisely because networking gets weird that a good representation at the type level could be useful. But I agree that it'd need to be done carefully to avoid creating usability issues.