When building an agentic workflow there are enough primitives that rewriting them from scratch every time makes zero sense.
What is a tool? How does the LLM understand the tool? Formatting a native function into a serializable input/output pattern makes sense to generalize and that does not need to exist repeated in everyones application code.
We use libraries to interact with the APIs themselves; nobody would say writing a spec-compliant API client was poor practice. Agentic harnesses are just one layer above: I need to call the API and I need to do it with certain expected conventions.