The creation of the zygote appears to be a somewhat arbitrary point in the process to select as the starting point of individual life, but likely one of the more convenient ones.
A zillion living sperm cells swim towards a living egg. The lives of those are all generally considered unimportant to people, especially considering that all of those zillion sperm cells minus one will die. They're recognized as about as individually valuable as the bacteria we kill with anti-bacterial cleaners or about as valuable as a skin cell or stomach lining cell.
When the sperm and egg merge, it's still a single cell with a high death chance, but genetically it's a full new human cell now. A human cell that has different genetics than the cells of the human mother, and different than any previously existing human being. So I can see why that's a fairly convenient point to pick.
Then later there are stages where the heart starts beating, or when it can first feel pain, or when it first starts moving, or when it's born and takes its first breath, etc. Those are other points where life has sometimes historically been considered to be new individual human life.