In my experience AI doesn't create the same abstractions as a human developer and you see this very quickly on a complex code base if you let the AI run wild.

Humans, and especially good developers, naturally create really good mental models for how to think about complex systems. We have to. That's ultimately how we build complex systems.

Refactoring (when it's not simply upgrading packages or changing languages) typically happens because we realise there's a better way to model the system we're building and we want our code to reflect the model in our minds.

AIs today simply don't do this. They write code that solves a specific problem, and while they do this well, they don't seem to create well defined and well reasoned models about how to think about complex systems. Or maybe they do, but the AI definitely doesn't say, "hey, I think what you're building is more like x than y, so I want to spend 20 minutes of refactoring your codebase".

I guess to use an analogy, imagine trying to tell someone how to build a bike who has never heard of a bike before, and therefore has no mental model of what a bike is. You'd probably start by telling them to put two wheels on a frame, then to add a seat on top, then to a add some pedals and attachment them to the wheel with a chain... At the end of this process that person might create something bike-like, and it might kinda do the thing you want, but it would likely be very different from the bike someone would build if they had a very clear mental model of what a bike is.

I suspect this is why current AIs rapidly hit a limit after the prototyping stage. I don't think task duration is the right way to think about this limitation. I suspect it's more of a proxy for how complex a task can become before the AI isn't able to create a deep enough model of the problem it's trying to solve, so begins to hit complexity limits and just spits out spaghetti code.

AIs are getting better quick though. I think they'll get there soon, but people are creating a lot of mess in their codebases in the mean time.