Nuclear exists.

It does, but the general public does not seem to believe anyone can operate it safely enough to allow it in their communities. That position may or may not be supported by facts, but that only matters in countries where politicians don't answer to the electorate.

Sure, nuclear may fit the bill. That avoids needing a new advancement if we're happy enough with the environmental impact of nuclear generation, but it doesn't avoid all the external costs beyond just the nuclear reaction.

Reactors themselves take a large amount of resources, some rare, to build. Infrastructure is another huge resource suck, all that copper has to come from somewhere. Nuclear has the nice benefit of being on-demand, so it does at least dodge resources needed for energy storage.