Because, as a person in it’s own right, it has rights beyond the its status of a chunk of whatever-you-want-to-call-it attached to, and dependant upon, the uterus in which it resides.
Row v. Wade established that a non-viable (for easy legal determination, that was specified as the first two trimesters of pregnancy) was a part of the woman’s body, to keep or expel IN HER OWN JUDGEMENT. If a fetus is determined to be a ‘person’ from conception, then it has the same rights as any other person, and the owner of the uterus in which it resides must respect those rights. At least that is my conclusion; I am not a laywer (Grace of Whatever!), much less one qualified to judge in the extremely rarified atmosphere in which such matters are decided - SCOTUS.
I strongly suggest that those who have never lived before RvW study up REAL fast on the coming nightmare - the NE coast and CA are the only places where there would be sufficient opposition to criminalization to ensure it not be enacted. The “Great Heartland of America”, let alone the former conferderate states, MO, IA, MN, WY, UT, ID, AZ would all be back to 5-7 years, loss of license, etc. - where I was 33 years ago. NM, NV, and CO are too close to call - they might just let their sense of independence keep them from running with the herd.
Good luck, and make sure you always can get to one of the coasts quickly. If the yahoos in Congress (the House is a foregone conclusion, the Senate is getting scary, and the President, Born-Again drunk that he is, would sign in a heartbeat - and if the SCOTUS has already overturned Roe, that would be a foregone conclusion, enact a ban at the national level, make sure you are never more than a week away from Canada. Or, ask your grandmother - the old girls knew a few (highly dangerous) tricks in their day - your mother probably takes RvW as a given, and has never thought about it. Otherwise, no fertile woman would have voted for the current crop of bigots running the US government.
Follow gay rights - they are now where pro-choice was before RvW, and are running into the same mindset we ran into. Vote accordingly - the bigots, assured of reversing Roe, are now ensuring that gays never have to RvW their kids will have to overturn.
Short form: Wake up!