It's not about complexity but architecture. Neural networks work in a fundamentally different way from imperative programs.

You can modify curl (a "tool") and predict how its behavior will change, with a high degree of certainty (bugs aside).

You can't modify a neural network in the same way. You can train it to behave in a certain way, there are no guarantees that it will consistently behave as intended.

More importantly, neural networks encode far more information and behavior than we can fully understand/identify - this is the fundamental reason why LLMs aren't just tools.