This can vary greatly from player to player, from campaign to campaign. An example: in one campaign a while back I played an undead hunter (ranger, cleric, a few levels from a couple PrCs). His backstory involved the death of most of his village due to undead - his survival mere chance. His drive to slay undead was such that so long as each adventure involved some undead slaying he continued on. He knew he couldn't kill all undead, but he strove to leave the world a better place - by slaying just about every undead he came across.
For another example, there have been a couple campaigns I've been in that have literally involved 'saving the world' (one kept a dark age of evil overlords at bay through various means (creating internal strife between them, killing a few of them, saboutaging some of their schemes, killing many of their minions, etc), while the other sought to help find an artifact that would eventually be used by a very high level NPC to prevent a dark demideity from ascending to true divinity (and, from what various oracles stated, eventually greater divinity). Both were a series of adventures that culminated at the final point of success, after which the campaign ended.
So, for myself, the campaign continues either because the over-arching plot has not yet been fulfilled or because traits of the characters would not allow them to rest on their laurels. They have some goal, some drive, that won't let them stop while some certain type of menace, etc still exists - even if truly destroying the menace in its totality is an impossibility.