I understand an annulment is valid when substantial evidence has been submitted and examined to prove the mutual consent was not entirely consensual. This may be a silly question but infant baptism is not consensual either, so why is an annulment reserved for the sacrament for marriage and not the other sacraments such as Baptism, Holy Orders etc.?
The Sacrament of marriage is conferred on the couple by each other. They are the ministers of the sacrament, and must have the proper intention, which is why if either one is not capable of giving the proper consent the marriage is not valid. Also, if either or both refuses to be open to having children, as being so open is essential to the Sacrament.
Baptism, Holy Orders, etc. are not conferred by the recipients. Marriage is