It matters for fairness. You can't get China, or any other country, to agree on low-carbon policy if they think it's unfair to them. So it matters very much that the US and other developed countries do more than China, India or other developing countries. Even if developing countries are currently emitting more carbon than them.

We're not going to get China to agree to meaningfully lower emissions without doing proportionally much more ourselves. That's the reality.

Plus all those historical emissions are still around, heating up the planet, so we still bear responsibility for them. What you're advocating for is making the last guy to use the bathroom clean it up, when in fact many people have been befouling it for weeks before this guy even got there. And we have a pretty good idea of who those other people were and what they did. If the objective is a clean bathroom, I guarantee you pinning it on the last guy will not get it done.

At this point I put anyone who says "But what about China/India/Nigeria?" in the same camp as climate change deniers. It's just a different excuse to pass blame and guarantee nothing gets done, which is the point.