Why do pro-abortionists argue about when life begins?

I personally know that life begins when fertilization occurs and the proof is in th foetus formed forthwith. Another proof is that if no one aborts the foetus a baby is born. Some people donot see anthing morally wrong in extinguishing the life of the most vulnerable humans-yet they oppose the death penalty. Human life is sacred and no arguement can change the fact that nobody has the right to take a life, to end a life, to stop a life from blooming. What do you think?