Language models would be fairly useless for most people if they accurately modelled the source distribution, no better than autocomplete. In fact, they were fairly useless when they modelled the source distribution, that's why ChatGPT was an instant hit whereas GPT-3 was mainly only interesting to other AI reasearchers.

What made LLMs suddenly interesting was that the responses were much more like answers and much less like additional questions in the same vein as the prompt.