The agent doesn't know what tools are available on your host. And searching for the documentation is slower and less reliable. It's an optimization.
the guiding promise of AI was able to figure out unstructured data. Manually crafting prompts and description examples for tools is a step orthogonal to progress. Searching for docs, great now you have 1000 tools saying they do same thing exposed to model, now we need another search on top of that?