Look at the other comments in this thread, the reason is more complicated than that. Open source tools might be fine but game engines can't be open source if they want to support console builds because that would disclose proprietary information.
> game engines can't be open source if they want to support console builds because that would disclose proprietary information.
Technically, they could. It would require someone who hasn't actually licensed the SDK, and so aren't subjected to an NDA, to reverse-engineer things and produce their own implementation under an open source license.
Certainly would be an enormous project, but it is well within the realm of the possible. It's been done with complex systems before.
> that doesn't make it a viable alternative for the games industry.
It makes it legally viable, in that it would allow the production of an SDK that isn't restricted by any NDA, and therefore could be incorporated into opens source projects.