Because then every function that could potentially cause a dns lookup failure needs to have that error code documented. Then ask yourself this, what's wrong with having (Out of Memory Failure, -1234) in addition to DNS Lookup Failure and HTTP Status codes?
Think of yourself as a http function. You know about the http protocol and therefore from your perspective a 404 NOT_FOUND is a valid result. However, you do not know about DNS lookups or memory allocation therefore if a problem occurs in those areas it is exceptional -> exception.
On the other hand, if you were a memory allocator function then returning out of memory instead of an address to the allocated memory would be fine. Because memory handling is your job.
> But open() in C does return a valid result on any of those billion reasons: (fd -1, errno reason).
Syntactically valid, but not semantically as -1 isn't a valid file descriptor.
