It’s because tools are mostly actions you perform and there’s a start and an end to it and it’s a concrete task with an output you expect to get quickly. Eg. Create a new resources, update a resource with id 123, read the resource with id 123, etc. Tools don’t allow for multi-turn interactions, whereas a separate agent can follow up with quetsions, clarifications and alternatives to solve a task.
That being said you still can have an agent as a tool for one-shot answers that don’t require back and forth communication. Things like “translate this block of text to Spanish”