This is one of my biggest issues with saving the world. Where is the help? At least LotR gave a reason why there wasn't an army of dwarves, elves, and men taking the right back to the mountain.
But most games it doesn't make any sense. "You are the troubleshooters." is not a good enough validation. The fate of the whole world lies in the hands of a few people and no one gives them more support? In a war campaign, if the PCs are going to destroy the enemy super weapon they should have ground support, diversion(s), air support, armor forces, overwatch, and transport/evac. Not given some more ammo and slightly bigger guns.
In a war campaign... there's a war on. The typical approach is simple - the bad guy has a huge army *and* a superweapon. The PC's side of the conflict can't just throw huge amounts of support to the PCs, because that materiel is required to hold back that army. The PCs, in effect, are their side's super-weapon.
In my own campaign, the PCs don't actually know that the world will end (well, actually just be ripped by such devastating war that civilization as we know it in North America ends - pretty much the same thing) if the evil plans are not thwarted. They just know that there's something nasty, and they're going to try to stop it. They don't know the full repercussions of what happens if they fail.