The context window is limited. Using half your context window for tools means you have a 50% smaller context window.

On a large and complex system (not even a mini ERP system or even a basic bookkeeping system, but a small inventory mgmt system) you are going to have a few dozen tools, each with a description of parameters and return values.

For anything like an ERP system you are going to have a few thousands of tools, which probably wouldn't even fit in the context before the user supplied prompt.

This is why the only use case this far for genAI is coding: with a mere 7 tools you can do everything.