A child having a guardian does not mean that both of the child’s parents are deceased. For much of American history a guardian had to be appointed even if the father was dead and the mother was alive. A guardian could also be appointed if someone giving the child an inheritance did not want a parent (usually the father) having control over the property.
An immigrant wanting to get married under the legal age would need a guardian to sign off on the marriage even if both parents were living overseas.
Do not assume everyone with a guardian had no parents living.