Well because it's not really a baby. It's just a blank templete of which we have a near infinite supply. If someday there is an existencial threat to humanity caused by rampant infertility I could see treating a pregnancy as something precious, but we don't have that problem at all.
My sister was a twin before she savagely killed her sister in the womb. If it has been the otherway around would I mourn the lost of my current sister who I love? No because I love her because of who she became, not who she was at 7 months. Any outcome at that point really would have made no difference to the fact that I have one sister. Her name would be the same. In fact despite their being 2 heartbeats and my parents picking 2 names, they have no clue which of those two ended up being my sister and who just became a bit of biological waste.