Here is a great article from a 2011 New Yorker: "Roger Ebert Wins the Cartoon Caption Contest." You should go read it. It's a delight.
Even great writers/thinkers like Roger Ebert have to generate hundreds of ideas before one lands. More to the point, even the great cartoonists of The New Yorker must generate far more drawings/captions than ever see the light of day.
There are several threads to look at here. The first is our old friend, Art & Fear's fable about the pottery class in which half the class would get an A for 50 pounds of pots, i.e., quantity, while the other half would get an A for one perfect pot, i.e., quality. The moral of the story, of course, is that the students who just generated tons of crappy pots (ABORTIVE ATTEMPTS) ended up doing better work than the students who tried to make a single perfect one.
The second thread is Lichtenbergianism's King of Hearts Fallacy: artists do not — and never have — made their art by "starting at the beginning, going until they reach the end, and then stopping." Never. You may think that The New Yorker's stable of cartoonists come up with the perfect pairing of idea and caption, do a couple of sketches, flesh it out, and then stroll into their editor's office with a wry chuckle to share next week's cartoon.
Alas, no. It does not work that way.
For example, check out The Rejection Collection, cartoons that never made it past the editorial team for one reason or other — mostly because for propriety's sake, as you can see in this small gallery:
For another example — and bringing us full circle — check out The New Yorker's caption contest, where cartoonists' wry little drawings are put out there with no caption at all and left to the tender mercies of people like you to provide 50 pounds of potential captions. All of that to produce one winner.
Lesson: make a lot of crap.