Because assault and drug abuse are illegal, and in the case of drug abuse, there is a addiction factor to take into account.
If this is both what the child and the parents believe they want, then the government shall make no law to restrict that choice.
Don't know if you have Jehovah witnesses in the US, but it's like them and Blood Transfusions, does the government have the right to force a Jehovah's witness to take a Blood Transfusion, even to save their life, if they believe it is the wrong thing to do?
Does the Government have the right to force someone to have or not have an abortion, even to save their or the childs life, if they do not wish to?
It's not just the subject that needs to be considered, it's the repercussions of it.
Edit: To take an extreme version of such a case, assume a mother is pregnant, and the child is healthy, but carrying the baby to full term will kill the mother, does the court have the right to 'protect' the child by forcing the mother to die? It's a different scenario from the one posted, but it does highlight the dangers inherent in the court feeling it has the right to make such a decision.