The next question (which is a great one, from what I understand) is: Why do LLMs use these phrases so much if humans rarely use them in written form? Maybe a fair portion of training data comes from lecture transcripts, where such responses are common when responding to direct questions? And/or system prompts are just instructed to be like that?
As far as I understand, it's due to RLHF. The reviewers the AI companies use don't necessarily know what kind of question is a good one, so when the LLM answers "That's a good question!", they tend to rate the answer higher because they like being flattered. Proxy models that are themselves trained on RLHF inherit this pattern. Similar effects contribute to sycophancy.[1]