Because even though we cannot save the world by ourselves, we can do it with collective action from all the citizens!
The world does not need saving. People do. The world is not in danger. People are (in danger). Or people are the danger? Maybe both of these are true on some level.
But the whole planet was fine without humanity, and it will also be fine after or if humanity disappears. By ignoring the climate change, people only hurt themselves (and indirectly the other living beigs on the planet, because they also suffer from it).
Very true, let's save ourselves then before we make it impossible for us to live on this planet!