Just my personal opinion on why this might be the case.

It's perhaps that people have hard time understanding the scale of the issue. They think, few inches so what?? but the downstream effects of it are catastrophic.

Even if as you are suggesting that millions of people might die, its hard to imagine the scale of that or how that might look like.

On the other hand, I have seen people far more passionate even about the lives/death of a single person. Maybe its the story telling aspects of it which plays a part in why a person cares about an issue or not, also how society reacts to problem.

A man looks upwards, if everyone else around him is running saying the sky is falling. He might assume so too, but if everyone else is acting the same nonetheless, (Sure people are speaking but Words are cheap and actions are hard and its actions of our society that we subtly pay most attention to.)

Maybe realizing this, I feel as if its a chicken and egg problem. We want people to care but people care especially more only if society at large cares about it but society is made up of people.

Going on a tangent, I come to this topic of chicken and egg a lot in my life. It feels as if things can be stuck and perhaps this also explains why things aren't improving, because if it did improve then I wouldn't be talking about it.

But because I am talking about this problem, it means that its usually involved of factors including societal/social dynamics and people are complicated and complex and its a hard problem to solve.

But just because it could be hard doesn't mean it isn't impossible. We can try to be helpless or doomerist about it but also Humanity has shown from time to time to do the (hard things) or even impossible things. Humanity is really impressive in that sense but also unpredictable and as such, I have given up the ability to predict the future but I have hope and trust within my heart that we could (together, as a society) fix this issue of climate change for our next generations because our generation borrows it from the next one and we should try to give it in good shape because why not/why shouldn't we.

I hope that we do, in fact, give a better future to the next generation and give them hope. Or certainly we can try to try our best in this possibility and perhaps be aware of the dynamics at play.