Merely because it ceases to be true.
Cling to it long enough, and not a doubt
It will turn true again, for so it goes.
Most of the change we think we see in life
Is due to truths being in and out of favour.
As I sit here, and oftentimes, I wish
I could be monarch of a desert land
I could devote and dedicate forever
To the truths we keep coming back and back to.
Robert Frost (from "The Black Cottage")
"What is known is rejected, because it is not sufficiently considered, that men more frequently require to be reminded than informed." -- Samuel Johnson
Megan McArdle discusses the same topic here, in context of self-help books, and personal finance, both of which tend to reiterate the same lessons over and over. Nevertheless, she knows that if a reader speaks of how much she helped him, he's either talking about a personal finance article -- or the kitchen gadget guide.
Which inspires me to philosophize on the topic myself, with regards to writing. Cliches can choke up a story in plot or setting or character -- and usually if in one, in all three because of their interactions -- but there's no shame in having a story with an old, old, old theme. At the very least, it has some staying power, as opposed to some new-fangled notion that springs up like a flower and may fade as quickly. It also dodges the bullet of being your own brand-new notion which you wish to teach under the impression that you really have come up with a brand-new and insightful idea that really does solve some of the great matters of humanity that people have broken their hearts over for thousands of years.
It's not just in matters of sagacity, but to put your wisdom in the mouth of a character purported to be wise is an excellent way to risk all. Some authors can pull it off with style, so that the character seems wise, but that generally requires dancing about the bush, to avoid committing yourself. If he actually has to commit himself, which is often wise (for the writer, I mean), ripping off someone else lets you get the wisdom and often enough the style so it sounds wise, too. If it's futuristic, he may admit to ripping off someone earlier. Come to think of it, if it's high fantasy, he can still admit to ripping off someone else, you just have to invent someone. Any character remotely describable as wise is not going to be ashamed to admit that he got an idea elsewhere.