In addition to Kosta's spot-on remark, there is a simple scriptural reason: "unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God" (John 3:5). Baptism is ordinarily necessary because Christ said so. The unbaptized baby does not have ordinary means of salvation applicable to him, i.e. baptism. He only relies on the extraordinary mercy of Christ, sometihng we surely hope for but cannot be assured of.
David was sure. Why can't you be sure?
2 Samuel 12:22 And he said, While the child was yet alive, I fasted and wept: for I said, Who can tell whether GOD will be gracious to me, that the child may live?
23 But now he is dead, wherefore should I fast? can I bring him back again? I shall go to him, but he shall not return to me.
In addition to Kosta's spot-on remark, there is a simple scriptural reason: "unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God" (John 3:5). Baptism is ordinarily necessary because Christ said so.
So the Holy Spirit of God is BLOCKED until there is a sprinkling? Does that make sense? This directly contradicts scripture, so I would like to ask how you would square this requirement against Eph. 2:8-9? Your answer cannot be that Baptism is a grace, because there is no element of faith in an infant Baptism, so that still contradicts Ephesians. In addition, under the Catholic view, it would not be "just" to forever penalize a baby if his family did not get him baptized before it was too late.