An untested hypothesis:
LLMs are fundamentally text predictors. There are many high-probability replies to “Tell me a funny joke”, so you wouldn’t necessarily expect them all to tell the same one. But perhaps, somewhere in the training data, someone published their conversation with an LLM in which they said “Tell me a funny joke” and it replied with the joke about atoms. Next-gen LLMs learn from this training data that if an LLM is asked to tell a joke, the probability-maximizing answer is that particular joke. So now they all start telling the same joke.
I would go even further than that—you don’t have to think you can do the job correctly. Some of my most fruitful projects started with me thinking something like