Well obviously. There are only roughly 50 million humans left on the whole planet immediately after the cataclysm, and you can probably knock off at least 50-75% of that over the next month, so of course we can’t do anything NOW.
So we hide, reorganize, and rebuild until we DO have the resources to do one or all of those things. It might take centuries or millenia, but vengeance is a pretty damn good motivator for humans. And it stays strong across generations too, as demonstrated by practically all of human history.
Might even rebuild faster if we use cloning or artifical gestation (seen plenty of mutated fetuses growing in tanks in labs, so that might be a possibility) to shore up our numbers, or manage to steal some particularly sweet alien technology.
It would be cool if you could iterate time passage between character spawns. So after one dies, you could set 50 years to pass, and the world changes some, human factions shrink/grow, settlements appear and expand or fail and decay, Zombie or monster spawns and concentrations change based on your choices, kills, faction interactions and strength, etc. Kind of like finishing Fallout, and then immediately playing Fallout 2, where some time has passed after the first game, and people have started to get their shit together.