Here is an excellent quote by Pierre Marcel on why we do not baptize infants because we assume they are regenerate. All italics are Marcel’s.
While recognizing that children of believers are baptized because they are in the covenant and are, as such, heirs of the promises implying a right to justification and to the regenerating and sanctifying influence of the Holy Spirit, a certain number of Reformed theologians have attempted to add one of the effects of the covenant of grace to the foundation of infant baptism, namely, presumptive regeneration. They have considered that presumptive regeneration could be the ultimate ground of baptism, more so than even the covenant. It must be acknowledged that this attempt has failed. Presumptive regeneration cannot be regarded naturally as the legal ground of infant baptism, for this cannot be anything other than the promises of God contained in the covenant. One cannot baptize on the basis of a presumption. To the question: “Why can you presume the regeneration of the children of believers?” one can only reply: “Because they are born of believing parents” ; or in other words, because they are born into the covenant. Besides, Scripture and experience afford proof that not all children born into the covenant are regenerated to salvation.
It is obvious that to refuse to consider this presumptive regeneration as the foundation of baptism is not at all the same as saying that it is impossible or unjustifiable to assume that the little children of believers are regenerate: we shall return to this point. But, in accordance with the indications of the Word of God, we do not wish in any way to restrict the divine liberty which acts in sovereign independence when and as it wills, and which is never confined to means. The promise of the regeneration of the children of the covenant is sufficient for us. It is not for us to define whether this regeneration in view of salvation is found in the elect children before or at the moment of baptism, or sometimes even years afterwards.