Novels are written by humans and read by humans, and as a result of that, the reader's default expectation is of human characters. The reader doesn't expect an explanation or justification for the default, but they do usually expect one if you diverge from it. It's the same with setting, for example. If you tell a story on an alien planet in 2459 or in a fantasy world or in medieval London, readers are going to expect that there's some kind of reason you're telling the story in that place and time instead of in, say, present-day Manhattan.
Novels are written by humans and read by humans, and as a result of that, the reader's default expectation is of human characters. The reader doesn't expect an explanation or justification for the default, but they do usually expect one if you diverge from it. It's the same with setting, for example. If you tell a story on an alien planet in 2459 or in a fantasy world or in medieval London, readers are going to expect that there's some kind of reason you're telling the story in that place and time instead of in, say, present-day Manhattan.