Higher level characters should have their eyes on larger issues. It is incumbent on a DM to have the scope of play change as the power level changes. The deeds of 20th level and higher characters should have more global ramifications than those of 12th level characters. Gods should consider those characters as either Cohorts or foils for their Cohorts. When Ruegel Dunbeard, 20th level Paladin of the God of Truth and Justice sets out for the Dungeon of Gazbarak-Dur, The God of Truth and Justice should insinuate what he wants into Ruegel's plans, that is Epic level play. Characters of such power get noticed by creatures beyond this existence and also any other characters of that level should start paying more attention to them. If they establish a base of operations, it should become a congregating place for lesser people who are of a similar ilk (not necessarily followers, but people who wish to live under the relative safety the hero's presence would provide, or act as carrion on the pickings the villain leaves behind. In short the scope of play must change as the characters approach "Epic" levels. While I used 20th level as an example, "Epic" is really a relative term. Dm's should establish what is "Epic" in their own world. In something like FR, 20th is not so unique, but their are worlds in which 15th is considered ridiculously high. The point is that the PC's as they go higher (particularly in games where the DM is loathe to put them in "final death" situations) become the movers and shakers in their region of the campaign world. If you want them to just continue to hack n' slash, or if that's what they want to do, then make the leading edge of NPC characters in the 30's or even 40's.
"It's not just that our inventions make things easier or take less time, I really do love the wonderful noises they make!" -Jebediah Jaspers, High Priest of Argon, God of Civilization and Invention