Um… Even as a metaphor, this doesn’t work, and it’s not presented clearly as a metaphor. Make it a simile, and it still doesn’t work, but for reasons that aren’t about grammar, etc.
The word tribe is not a proper noun; don’t capitalize it just because you (correctly) capitalized Amazonian.
Galaxy is not a proper noun, either.
Earth is the like an uncontacted Amazonian tribe to the rest of the galaxy, and aliens don’t want to contact us because they want to preserve our primitive and unique culture.
(Yeah… It’s kinda insulting when you’re on the receiving end, isn’t it? Think about that, folks.)