The Apple CPU's architecture is already known (ARM). To iterate from there is probably easier than designing a modem without the basics already designed and ready to go.
Apple also has a pretty robust ability to dictate to their developers how and what they must do to run on MacOS, and they produce the compiler and toolchain as well. They seem to do a lot less shimming and other compatibility work on behalf of developers. Modem doesn't work that way, there's tons of existing, immutable hardware across the world with its own quirks and edge cases, all of which you have to support seamlessly.