Not really a good metaphor. Steve Jobs, Tony Fadell, Elon Musk, Max Levchin, and many more entrepreneurs chose to start a second company rather than take a C level job somewhere else. A better analogy is whether to adapt an off the shelf piece of code to scratch your itch (often a reasonable choice) or to write your own from scratch, clean slate. The latter is much more risky, but that fits a certain personality.
Building a successful company is not a joke even if you have knowledge and connections. Users don't value what and whom you know.
At her level and for all she has done, getting to be the CEO of a billion dollar firm is actually a great deal.
Looking at it from other way around, Her career at Google might have probably come to a halt given the fact that Larry and Sergey are going to be there around for a long time to come. On the other hand, she could not be the CEO of a successful firm- Because such stints demand prior experience.
In short the change she got now is because, the company is currently a underdog. No big CEO who is already on a wave of success will take up that kind of a job. She needs the company because she gets to be the CEO and nobody else would offer her the same position.
