So many?
Not that many though.
Doing a thread search for 'banjo jokes' five threads came up with a total of only about two hundred posts ;-)


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I think it will be to do with trying to lighten a painful experience.
I definitely find it more painful on the ears to hear a banjo being tuned than a guitar being tuned.
I find it more painful on the ears to hear some banjos being played than possibly any other instrument I've heard.
We know that banjo players are pretty tough and imagine that they can take a bit of ribbing.