That's the idea. Writers show us our possible futures, and it's up to us to pick which one we want to implement. Dystopias are cautionary tales.
> I only read stories to my kids that have good old-fashioned happy endings.
That's actually kind of a modern development, itself.
Dystopian pessimism is more sinister than that, I'm afraid. The spirit of pessimism is the idea that what ever you do, things go wrong, because we're bad. So there's no point trying to implement things. Whereas in optimistic fiction things go bad but the protaganist learns something and sets the world to rights by the end.
>>I only read stories to my kids that have good old-fashioned happy endings.
>That's actually kind of a modern development, itself.
Good point! Makes me think of Grimm's Fairy Tales, Aesop's Fables. Must take another look. (If I recall correctly these are moral tales rather than pessimism, however.)
That depends heavily on the dystopia. In Perdido Street Station, for example, the characters do create meaningful change, though I can't imagine anyone considering that book uplifting.
I take your point, though, a lot of dystopic fiction does end with no progress. But I think that's not necessarily a statement of the possibility of change: we usually see the dystopia through the eyes of a single character in a short slice of their lives. It's a very American business, quarterly profits way of seeing things, to measure change in a single character's actions and vision. Change can be slower and larger than that.
If authors really believed this, why would they even bother writing?
Consciously: perhaps trying to impress other authors/screenwriters of the same ilk. Perhaps trying to make money; to be famous.
Unconsciously: perhaps trying to project their despair onto the public. By attributing the feeling of despair to an external cause (which they help to create!) they get relief because they don't have to acknowledge and face it internally. Crazy, I know, but ultimately evil is unknowable and it's better not to dig too deep.