Not sure why I was downvoted for this, but it doesn't matter.
If you want your game engine to be useful, it needs to perform well. It doesn't matter if your engine is home grown and only for your application, or if it is something you license to other developers. If it is to be useful, it needs to perform well. So, they do. Game engine developers are not stupid.
If you want your game engine to be useful, it needs to perform well. It doesn't matter if your engine is home grown and only for your application, or if it is something you license to other developers. If it is to be useful, it needs to perform well. So, they do. Game engine developers are not stupid.