Why do coding assistants struggle to do the bare minimum?

Whenever I use an editor agent for a quick simple patch it always tries to turn it into a massive engineering project.

Instead of just editing the single line that's broken it hallucinates patterns and abstractions we don't even need in the project.

It makes code reviews take ten times longer than if I had just fixed the typo myself.

What kind of system prompts or strict rules are you using to force these agents to stop over-complicating things?