This is related to a wider myth about artists, writers in particular. Standups are "wordsmiths" too, and they often portray themselves as bemused observers of the human condition. As a result, many people want to see their one-liners as not just true and funny but profound as well.
Then there's the link between absurdity and humor. It's often made in spiritual teachings like Buddhism. Such doctrines say it's a sign of maturity and wisdom to accept that life is absurd.
The Dalai Lama himself is seen as something of a comedian. This is not so much because he makes others laugh (although I'm sure he does from time to time). It's more because he's often seen laughing himself; laughing mostly at the absurdity of life. And if anyone has a right to be known as unquestionably wise, it's the Dalia Lama, right?
Sure, the guy has learned a lot. And he has much to say that we can all benefit from. But it should also be remembered that he's a politician. Much of what he says and does is focused on practical goals for himself and his people. As a result his wisdom is not completely selfless and pure. And if it' contains these earthly motivations, it must be a little diluted, surely.
But back to comedians: I think this belief that they are wise is not unlike the same characterization of old people. Certainly some codgers are wise. But let's face it, a lot of them are just bitter and regretful!
I've known a lot of comics and I don't think they're any wiser than the general population. Pretty much everyone learns from life, but most people just don't have the desire to get up and tell people about it -- least of all through the medium of jokes.
I think it's useful to see comics as artists more than thinkers. And art has value because it holds the mirror up to nature. Comics, unlike other artists, hold up a fun-house mirror. There's definitely something valuable in the whole process. It gives great pleasure and enjoyment to the audience apart from anything else. And that's great. It's a mistake to see it as profound.
This image of comics as great, deep thinkers is best illustrated by the way people revere Bill Hicks. He was certainly a great comic. Very funny, unique, amazingly confident and accomplished and at such a young age. But I wouldn't say he had any great wisdom.
He was a seeker of wisdom, sure. For example, he used to take magic mushrooms in search of higher consciousness. And no doubt many people see this quest of his as qualifying him for this role of "philosopher king". But really I think that anyone who thinks they'll find wisdom through drugs is misguided -- even foolish.
The fact that he died so young has added to his mythical status. Like with so many other figures such as JFK, James Dean, we can retrospectively attribute qualities that we don't have to test with reality. So we can see him as wiser than he actually was.