Because for at least the first 3 or 4 months, it is not yet a person, just a potential one, and because being forced to undergo the physical, emotional and mental strain of pregnancy and childbirth against one's will is extreme torture, not just "going along with the flow".

Women who want to have a child willingly undergo this torture, just as people who want to train for a marathon willingly undergo physical pain, but if you did not chose to do so willingly, it is a form of torture, and abortion a form of self-defence.

No one is morally obliged to go through torture to save someone else's life.