Actually it's "has an ability to sustain existence" which it does. Viability is not a precondition in the definition.
The real reason that there isn't a scientific consensus as to when life begins is that it's not the way biologist look at life. We don't see life as a start/stop point. We see it as a long evolving process over vast periods of time. We see life as a continuum. Even in individual sexual reproduction there is no dead phase. There is no non-life phase. The sperm and ovum are alive, the zygote is alive, the fetus is alive, the mother nourishing the fetus is alive all parts of the evolving process through viability are alive. So when does life start when all parts of the life process are alive to begin with? Science can't really say cause it's all alive to begin with. At no time during this process is their a dead point or non-life unless that is...you're dead. Then that particular process has stopped but the continuum of life goes on. So from a scientific standpoint it makes more sense and is more predictable to model life as a continuum than as a series of stop, start points, which it really isn't.
It's like asking 'When did planet Earth begin?" Astronomers and Geologist can't really give a definitive answer cause there was no specific start/stop point. It was a long evolving process and the planet now is not the planet it was a few billion years ago. Earth just didn't happen in one specific moment.