Maybe its just my reading preferences, but I don't know that I agree that they are optimistic generally speaking. A lot of what I have read tends to be either picking up the pieces from the fall of humanity or a warning on human arrogance and hubris. Usually you have some kind of humanity ending issue that gets resolved but usually has an element of sacrifice or loss in getting to that resolution. Ive always found sci fi to be a "this is how terrible it could be if we dont hande X issue correctly, but at least there will be people like the protagonist who will try and do the right thing". I think if you took away that optimistic resolution to some of these plots, they wouldn't be very compelling as a read. I think to more directly answer your question, these stories are often trying to make a point. Some of the ones I've enjoyed most tend to do a better job of incorporating the death by a thousand cuts you describe but are ultimately trying to say something about a specific issue. I think its also hard to argue that our modern time isn't better than the past. That's not to say its perfect or that there isn't suffering, but if we were to describe our lives to someone from a thousand years ago, they might feel that our current world is fairly 'optimistic' as well.

This could definitely be some bias on my choices though so grain of salt lol.