Because a human can be biologically alive while lacking consciousness: individuals under anesthesia or in a coma are still alive, yet their consciousness is temporarily removed. When people speak about “life,” they sometimes use it to mean “conscious life” or “sentient life.”
This is a valid concept, but ultimately arbitrary, as there are different levels of consciousness or sentience and it would be difficult to define a boundary for when someone has sufficient consciousness to be considered “sentient.” The same concept applies to a fetus, who may technically have “human life” but not sufficient consciousness to be considered sentient.
We'd have to distinguish between "alive" and "a human person." Something can be alive, but not a human person. A single skin cell is alive, but it's not a human person. Most people would argue that something or someone becomes a human person when they obtain the capacities necessary for consciousness: sentience (the ability to feel) + sapience (the ability to form complex thoughts and comprehend one's environment). So in the case of an embryo or fetus, it wouldn't satisfy these criteria, and thus wouldn't be a fully morally significant human person.
Idk as to how much you subscribe to these..
Now we're asking how can someone be "temporarily" dead, in the moment they either are or aren't?
I like these arguments since it helps to properly comprehend how to present someone with the distinction that while both are human bodies, but one is alive and the other isn't, and one can be of more value (the fetus), its still a matter of personhood and potentiallity to deploy that personhood.
If I have a dead human, and keep its body artifically alive even if the brain has permanentely ceased its ability to deploy consciousness, no one would regard it as a person. Same way a fetus can be kept alive, but not granted the moral consideration we give to people.