You can, and indeed that's what we JDK developers do.

But even aside from the fact that writing the code is only a very small portion of the effort in this particular project (you can see that the volume of code making its way into the JDK is very small compared to the number of people involved), I'm often amazed by the gap between how well a frontier model (GPT 5.6 Sol in my case) can comprehend code and investigate a bug, and how badly it writes code and documentation, even when it understands things well. So not only is writing the code not a large portion of the effort in this project, it also happens to be one of the things current models don't do as well as other things.