The main issue is that if you ask most LLMs to do something they aren't good at, they don't say "Sorry, I'm not sure how to do that yet," they says "Sure, absolutely! Here you go:" and proceed to make things up, provide numbers or code that don't actually add up, and make up references and sources.

To someone who doesn't actually check or have the knowledge or experience to check the output, it sounds like they've been given a real, useful answer.

When you tell the LLM that the API it tried to call doesn't exist it says "Oh, you're right, sorry about that! Here's a corrected version that should work!" and of course that one probably doesn't work either.