It was probably just easier to implement. The build script[1] already has the source code, extracting the number from a comment is trivial, while retrieving out-of-band data like Caller ID from the fax server is likely more complicated. For a joke it's not compelling to do that, especially if you've already been fighting the fax server...[2]

[1] https://github.com/lexbailey/compilerfax/blob/main/build_and...

[2] https://github.com/lexbailey/compilerfax/tree/main?tab=readm...