More than half of the committers on Chromium aren't Google employees. I assume the reason they haven't gone off and forked it is because they have no reason to.
They probably haven't forked it because they know it's a futile effort to maintain and develop something as complex as Blink. Anyone who's ever worked on a large project like that knows very well it's easier to start from scratch, even if it's going to take a herculean effort.
