I think Apple would have to publish "proprietary" hardware information to make this possible. Currently, people spend significant amounts of time reverse engineering the hardware to get software working with it.

On top of that, there's a cost of open sourcing besides revealing proprietary information--you generally need to audit and cleanup your code base (cleanup commit messages, code comments, etc) that could be, at best, embarrassing, or, at worse, reveal product flaws/bugs

Edit, looks like I was beaten to the punch https://news.ycombinator.com/item?id=25728930