Why is a very good question indeed! My guess is we have not figured out how to include long term enough tasks into the RL training.

As to whether they can today, it's pretty easy to figure out that they can't by observing the difference between asking for very high level things like "please make me a website that will be very successful" vs sending it a detailed plans on the goals and many of the specifics and then holding it accountable towards the planning.

Doing appropriate project structure and trying to optimize code quality are definitely good methods to try and increase the chance that the project will succeed long term but I would say they are only methods towards the long term goal and the LLM need to be thinking of the long term goal and how to plan towards that to achieve a much wider and subtile range of methods to achieve long term goals.