Because if we aren't causing it, then there's very little reason to make vast changes to our way of life in hopes of preventing it.
That's a misconception you're holding onto. Those working on mitigating carbon in the atmosphere don't' care what the source of the carbon is. Whether we caused it or not simply doesn't matter.
> If we didn't cause it, the whole edifice crumbles.
The attempts to stop a calamity are worthwhile regardless of the original source of the calamity. Engineering the environment does not depend on whether we think we accidentally engineered it before.