Just another abortion question

Sorry to start yet another abortion topic, but I have a question that's been bothering me for a while. This question is NOT meant to shame anyone.
People keep saying, "if we make abortion illegal, a dead person will have more rights to their body than a pregnant woman will", because someone can decide what is going to happen to their body before they die, but if abortion is illegal, women can't make certain decisions pertaining to their body.
My question is, how is this a good argument? The dead person is dead. They have no idea if their wishes will actually follow through once they've passed. A fetus is alive, whether you consider it to be a person or not, it's still alive. Why does a dead person have more rights to their body than a living fetus has to theirs?
The dead person loses nothing if their wishes don't follow through, but the fetus loses everything.