Why is the phrase as popular as it is?
On another note, I also don’t quite understand what it means or implies. When someone doesn’t have faith in humanity, what does that mean?
Also, what is the humanity in the phrase? Do they mean all 8 billion people? Or is humanity understood here as something metaphysical?
Has nothing to do with OPs question or my answer, but to address a single tangent you’ve offered, yes, people are complex.
OP asked what it means to not have faith in humanity, and the person who responded to your comment had a nuanced take on the answer. Is that really a tangent?
(btw your top comment is a very good answer)