Because life is more complex than you allow.
To the contrary. Life is incredibly complicated and neither I nor the State know what a woman is facing. So it should not be for me or the State to tell her if her circumstances justify an abortion. We should remove legal obstacles so that she can make that judgement for herself.
I imagine your 'not even in the case of rape' phase was only possible because you wouldn't or couldn't look with empathy at the reality of a person in that situation. Now you either can't or won't acknowledge the humanity of a full-term child.
It's not about empathy or feelings. At that time, I was lexically prioritising the baby's life over the woman's bodily sovereignty. If you allow a rape carve-out, you have to ask how the baby somehow magically matters less if the woman was raped than if she wasn't. At that time in my life, I was waking up to having been in an abusive relationship in which the only reason why I wasn't pregnant through rape was a Mirena. I was waking up to the idea of women having rights, like the right to say no to sex and to unwanted things in general. Realising that women have the right to say "I don't want to be pregnant any more" kind of followed on from that.
The 'lost future' comment is not an argument per se.
It came across as the "you could have aborted the next Mozart" that, even when I was on the anti-choice side, I rolled my eyes at.