Because there isn't a "point". It's the Sorites paradox: how many grains of sand does it take to be a "heap"? A hundred? Ten? One? The paradox arises because the "heap" is vague. "Alive" is similarly vague.

A cell is a complex thing, with a lot of redundant parts constantly in motion. You can remove many different subsets of them and it will still work, still be "alive". There is no singular point at which it passes from "not-alive" to "alive".