One of the big arguments is that we don't force people to donate blood or organs. Even for someone who's at death's door without a transfusion or transplant. Even for parents and children. We don't do it, it's beyond all medical ethics.

For the same exact reason, you can't force one person to serve as another person's life support device. Even if the fetus were a full human being with fully equal rights, which is a stupid argument on multiple levels, that fetus has no right to force another person to act as its host regardless of their relationship.

Otherwise, prepare for forced organ donation and blood transfusions.

Pregnancy is dangerous, it was one of the most common causes of death for fertile women historically. Even today it results in permanent body changes with a big chance of disability and death. Permanent incontinence, fistula, diabetes, you name it. Abortion is much safer for the body than pregnancy. That's another reason it's unethical to force people to give birth against their will. Naturally, once the fetus is able to survive on its own, we no longer allow the "host" to arbitrarily kill the fetus, hence the restrictions on late term abortion.

Anyway, hopefully this helps you to understand the reasons a lot of people think a pregnant person should have the same rights we give all other people.