To answer your questions, one need only look through the lens of incentives. What are people motivated to do?
> If we are really on the brink of an existential crisis then why haven’t we been panic-building nuclear plants for decades? ... nuclear has been widely panned amongst the environmental cadres for decades.
Making nuclear power into a bogeyman was the fossil fuel industries' first and most wildly successful astroturfing campaign. Co-opting the nascent environmentalist movement in the 1950s was a stroke of genius.
The incentives for fossil fuel miners are, I hope, obvious here.
The incentives for politicians are about keeping their jobs. Say things to stay visible in the media; do what your donors want. Those people in the future? They won't vote for me next election.
So yes, you are right in that politicians are saying things for political gain. Scientists and engineers make really bad politicians though; they tend to say what they believe to be true.
For politicians you could add incredulity, inertia, and distraction as well if you wish.
Incredulity: they lack the skills to evaluate what scientists say, so they rely on heuristics: the climate is stable. Inertia: meetings and more meetings. Distraction: there are always more visible and understandable "crises" (really, bike-shedding) that serve equally well or better.
And narcissism too: no-one honours people who prevented crises; they remember the hero of the hour. (See: the widespread belief that Y2K was a big hoax, rather than there being a day of gratitude that the world's financial systems didn't seize up for 18 months.)
My expected scenario is this: panic after a few really large weather events in the 2030s (say, 10 million or more dead in a prolonged heatwave, or a bunch of them, and/or simultaneous crop failure--down 40% or more--in 3 of the 5 major grain-growing regions of the world, twice in five years, and/or really serious flooding in major urban areas).
I expect to see the beginnings of serious government action in the 2040s. If I'm still alive.