Jürgen Hubert said:
And once they got rid of the tyrant, what then? He might been the only thing holding his nation together - so if they don't take charge of it, it will dissolve and start an ugly civil war that will cause even more deaths. So what are they going to do about it - other than ruling the nation themselves?
My point is that once the PCs become powerful enough to defeat whole armies by themselves, they should think in grander terms than just slaying bigger monsters. They have the power to shape the world for good or ill - so at the very least they need to come up with good explanations for why they aren't doing just that.
You know, the basic question that needs to be answered is what exactly you are aiming at with this thread, because I could simply pose the opposite question "Why should they if they don't want to?" and describe a counter for every argument you might bring up. This could turn this thread into a morass of back and forth that won't lead to any real result.
So what is it you're after that hasn't been mentioned here already?

The very simple answer to your question is "Because all players involved don't want to." And basically, that's the only thing that counts, the intentions of the players in question about what they want in their game. The repercussions of the characters' actions are only important if a) the DM brings them up and b) the players think their characters care about them. And really powerful characters have the luxury to CHOOSE whether they care about the repercussions of their actions or not. There isn't much that can make a group of 15-20 level characters take responsibility besides DM Fiat, and in the case of uninterested players, the game will most likely disintegrate if the DM tries to force stuff on them they are not interested in.
Contrary to what was already mentioned, D&D used to be not only about killing monsters and looting the bodies. At least in earlier editions, the perspective of characters as rulers and political world-shakers was built in as well. It's not so much anymore though, and the speed of the game has simply increased so much that it's no surprise people want to play their high-level characters like they are just extra powerful dungeon-crawlers...after all, that's what they ARE. If it took you 3-5 years to level a character to 15th level, you had more than enough dungeon crawls and monster slayings that you were happy to try out something different with your character, like managing a kingdom on the side while running a new 1st level character through the Caves of Chaos. By now, you're supposed to be 20th level after a year or so of regular playing...and in combination with adventure paths and high-level adventures that are nothing but high-level dungeon crawls, that simply changed how players view their characters.
It all boils down to what the players want, and that's not simply going to change by pointing out to them that their characters are something like superpowered demi-gods for that setting now, and should take a broader view. If they don't want that, it ain't gonna happen.