Becoming principal engineers is usually just playing the cards right in general. Very rarely does anyone get this far on technical merit alone. I know several people at Google who easily deserve the title based on quality and volume of their contributions, and several "principal" engineers who do not. Life isn't fair, what can I say. In addition to being good, you have to be at the right time at the right place, please the right set of people, and "play your cards" in other ways. In fact, if you do the "soft" things right, "being good" at technical stuff matters very little, beyond a certain basic threshold.