This might all be me overthinking stuff, but the thought's been nagging at me for a while.
If a campaign contains some threat which risks destroying the entire world, how would you address why stronger heroes wouldn't travel from every land to bombard the danger with spells like Wish or Power Word Kill? Hard to imagine that even the most selfish legendary fighter would refuse to help if the enemy is a threat to all life on the planet, including them.
With reasons for why deities don't magically fix everything it's always possible to handwave something, but what about other mortal heroes? Is every last one of them convieniently engaged with a similar but more dangerous extinction threat which demands their attention? Have they all been killed while defeating their own enemies the day before the players need them?
The only excuse I can think of that doesn't seem ludicrously convienient is a stronger adventurer assessing the threat and assuring the PCs that they can handle it. I would hate that as a player, because knowing that there was someone who could easily solve the problem but didn't think it was worth their time would take all the stakes out of what was happening.