New Vegas consistently has a much larger player count than 3 on steam charts. Fallout 4 and 76 do have more, but it's still impressive given it's age. NV has an average player count of over half that of 76. I think it's popular enough that calling it a cult classic isn't really reasonable.

Totally up front I'm just a big nerd and I don't mean any of this with any hostility, I mean we're talking about a game. I just find it difficult to wrap my head around how, even limited to Fallout 3 and 4, you drew the conclusion that Fallout's themes are about "you become the god that saved or destroyed the wasteland." Yes, in 3 the main quest has you either fixing or sabotaging project purity, and the game's "moral" choices are typically extremely black and white. So much of the game, however, presents more complex ideas and scenarios. It represents so many communities struggling to survive in an inhospitable land, like in Megaton where people are desperate enough to live in a town surrounding a nuke. You don't become a god and either save or destroy the wasteland, individual choices in individual communities leave either good or bad consequences for those people. 90% of the actual game isn't this one big "save the world" plot.

Or in Fallout 4, you're constantly being asked somewhat more difficult choices. Like joining the Brotherhood of Steel being common in players who played 3, only to be presented with the choice of what to do with Paladin Danse when he realizes he's a synth and you're ordered to kill him. Or choosing whether to make use of "the sight" by providing that lady with drugs against the wishes of preston garvey. An entire element of the game's mechanics revolve around building up settlements and creating the infrastructure to house and protect people. There's so much commentary on human nature in these games, and in the stories they tell, y'know?