They want to keep them legal because of cases where a baby becomes a threat to the mother’s health late in the pregnancy. It’s more of a “just-in-case” clause, though it could be worded more clearly to express the purpose.
If someone wants an abortion, what reason would they have for waiting right until the last moment? Wouldn’t it make more sense to have it earlier?
I’m not saying I support full-term, just trying to understand it