Because one of the most fundamental rights is the right to bodily security and bodily autonomy. Continuous consent is needed for one person to use the body of another; denying the right to end a pregnancy denies these fundamental rights just as much as, say, forced medical testing or forced organ donation.
You are a human being, conscious, and entirely capable of expressing your will to live. Despite this, if you needed your mother's organs, tissue or fluids to live and she refused, you would still have no right to them even if you would surely die without them... because your mother - like every person - has the right to bodily security.
... and your mother still had this right even when you were a fetus.