Quite simply, there is nothing inherently "depressing" or "disappointing" with how people happen to be

"Depressing" is a 2-place predicate - Depressing(x,y). A certain situation x may or may not be depressing to a certain individual y. The situation that humans are both uncharitable, selfish and furthermore deluded about that is depressing to me.

Causally, this is because I also used to be deluded about it, so finding out that people are not as nice as the propoganda says they are feels like a loss, though, as you point out, it is not.

But the fact that the causal explanation for my disappointment in humanity is that I used to be deluded does not logically compel me to change my standards.

Indeed, I think that it is precisely because we are mostly deluded about what our own typical behaviour is, and what our typical motivations are that we even have a concept of goodness. Our concept of goodness is what happens when we believe our own bullshit.