Because they're not aware of what user wants, and they need to know what expectation is, if it finds out it's a famous open problem it may think informing the user and not trying is best option as average user may not prefer it spending hours when success isn't guaranteed, by telling it to use it's available tools and not stop at partial progress, use subagents for various independent approaches it's allowing LLM to know what it should do and what counts as success. These things do great when goal is well defined.