At low levels, monsters encountered generally have little or no treasure. A few copper or silver pieces, a small gemstone here and there; the goblin chieftain might have a +1 mace, which when you're first level is an exciting find. But if you've got a party of 10th level characters, say, fighting giants, and they go through a normal on-level encounter, the commensurate treasure reward is somewhere in the thousands of gp; suddenly even the trash monsters are just these enormous piñatas full of gold pieces, waiting to be smashed open and have the loot come pouring out.
Why is this? Well, because high-level heroes need high-level gear, which is made incredibly expensive to keep it out of the hands of low-level heroes and thus muck up the game balance (beware the 2nd level fighter with a +5 battleaxe). And also because, depending on the particular group, why would you go through the risk of daring the demilich's Tomb of Gotchas if there wasn't the possibility of walking away with a diamond the size of your head at the end?
This leads to situations where you're fighting grunts wearing +2 chainmail and carrying vorpal swords, not only to bump up their threat level against the heroes, but also so that if defeated, the loot they drop can be taken back to town and sold (because the heroes are still wearing the +3 mithral plate they got from that last dragon's hoard, so they might as well sell the +2 chainmail). But of course, that requires there can be somebody in town who has 1,000 gp to buy the +2 chainmail ... but that person is probably a 5th level expert/noble (in 3.x) or even a 0-level character (in previous versions). So ... dang! Why are we wasting our time with these giants? Let's just ransack the town! Ack! That means the merchant has to have some way of countering 10th-level Chaotic Troublemaker PCs ... but if the merchant (or their bodyguard) is all that, why aren't they out there stomping the giants?
And so on.
Of course, the most popular and time-honored way of dealing with the issue is "Don't look behind the curtain." Establish an unwritten agreement (or even a written one if you like) with the players that This Is the Way the World Works, and that you'll continue to provide the monsters and treasure as long as they agree not to go ransacking the campaign city or otherwise turning the setup against itself.
Another possibility is to continually move the campaign city to a higher and higher bracket ... from the Village of Hommlet to the City of Greyhawk, from the City of Greyhawk to the interdimensional city of Sigil -- where people on the street do routinely carry vorpal swords because the economy is just that huge. Personally, while I see the value of this idea as a problem-solving tool, I hate it from a storytelling perspective. Planescapey stuff makes my teeth itch and is completely the kind of fantasy I never want to touch with the proverbial 10' pole.
My personal favorite way of dealing with it is "never go above 10th level," but certain corners of the gaming table are chafing at that prospect. ;P I have been observing LotRO to see how it copes with "super-high level characters in a generally low-level world," but honestly I don't think it's done that good a job. Outside of the instances, it's still okay and mostly-resembles Middle-earth, but you start going on those end-game raids, you might as well leave Professor Tolkien at the door because nothing in there makes sense from a storytelling point of view. It's all game mechanics.
That doesn't even begin to deal with high-level cheese like the good ol' "scry'n'fry" teleporting ambush and character death-and-resurrection being an expected part of the adventuring day. But I'll get to that rant later...