One of the reasons for that is that it is not the current SC's role to undo a prior decision unless the case before it is written specifically point for point to undo a past decision (not just a part of the decision). And the person bringing the suit has to have "standing"—in other words, can show harm. This is a hard case for a fetus to do, since the left has been arguing against Personhood for decades. Some states recognize the fetus as a person and some do not; for instance, if a pregnant mother and fetus are murdered, is the perpetrator arraigned for one murder or two? Depends on whether the law in the state where the murder took place is applicable.
Much of the problem lies with the original decision, which was in darker ages medically. Since Roe, medical science has advanced to the point of recognizing that a fetus feels pain and that the heartbeat is present earlier than what was only dimly understood when Roe was decided.
And the original case violated the norms of small-r republicanism; it should never have been a Federal case. So without having read every word of the latest decision, it sounds like the Justices mainly sought to correct the unconstitutionality of Roe having been made a Federal case in the first place; but may not have waded into the Personhood question. I could be wrong. It depends on the wording of the Dobbs case as to whether they were even asked about that. Does anyone know?
Do you really think they have to have a case to actually say babies are humans and not dogs? Well then we maybe should just call babies dogs then animal rights activists would shut them down.