Because writing prompts is hard.

Most AI coding tools can generate a Todo app from a small prompt. This is because that problem is well understood.

When you try to use AI coding tools on your own projects you need to start writing a prompt that teaches the AI about your current architecture and decisions.

So the initial prompt is large.

Often the task needs knowledge of other files in your project. You can add them by hand or some AI tools will search the code base.

The prompt is now huge.

When you run that prompt you may or may not get what you expected.

So now the issue is how much time do you spend getting the prompt correct vs just writing the code yourself.

This area is brand new and there are very few resources on how to use AI coding tools effectively.

I have yet to see one demonstration of effective AI coding tool use on a project of reasonable complexity.