1. There's too much money directly opposing the goals of fighting climate change
2. Humans aren't big on alignment. FFS, we still don't have good UX/GUI for Linux and it's mainly been stagnant* and divided into Gnome and KDE for 20+ years.
* despite the impressive accomplishments, it's essentially true
2. Your example isn't something life or death. Humanity has aligned on numerous things and changed significantly amd uniformly during the past 30 years without even trying. Why is it so hard to come up with a list of demands and reforms for everyone to agree on and march behind to save the world?
Climate change has been a mainstream issue since the mid-90s. From then until now we've had massive material and ideological changes across the globe, none of which required the support or awareness of humanity or the voting public.
The internet, cell phones, social media, fracking, AI, multiple wars, changes in attitudes towards family, sexuality, and marijuana usage have all occurred during this time period. Why then has a slight, gradual reduction in greenhouse gas reduction been so hard?
Changing that cost money—and people resent having to pay more for change that they don't perceive will make much difference to their lives.
Whether they're right or wrong doesn't alter that.
Governments have been unwilling to push those changes because they are unpopular and expensive. Governments have also been unwilling to impose hardship on their people if other countries aren't going to do anything. A lot of people, in this thread, don't think it will be bad, or that it will happen after they are dead.
In terms of making a plan, what is the point of making detailed plan if nobody is going to follow it? Also, there are lots of things that we don't know how to do yet, and can only plan to investigate.