That, and the hardware relevancy, seem like plausible reasons: but why limit yourself to only the rock-star engineers willing to work in secret and for extreme

That, and the hardware relevancy, seem like plausible reasons: but why limit yourself to only the rock-star engineers willing to work in secret and for extreme hours? It doesn't seem like they had competency, competitive, or budget constraints that would have prevented them from using more engineers, more openness, or, again, more engineers on the project.