Circumcision and baptism are similar because they are both outward seals of their respective covenants. The Old Covenant was based on the nation of Israel. If you were born into that nation, you were therefore expected to be circumcized as a sign that you were bound by the covenant.
The New Covenant is not based on your ancestry, but based on belief. Baptizing an unbeliever would be pointless, for without belief the baptism symbolizes nothing. I don’t think infants or small children can really grasp what is required of them for belief.
I’m not sure that argument has enough validity. Circumcision was an outward sign of being bound by a Covenant with God, not just to show that you were an Israelite. The baby being circumcised did not have the capacity to understand the Covenant. What if later on he rejected God?
Infant baptism is an outward sign of a seal of the New Covenant. Certainly that infant may grow up and reject God as well. In the Acts of the Apostles you find stories of entire households being baptised. Those households almost certainly contained infants, or toddlers. Nowhere in the bible does it say (paraphrase) “And we baptised entire households, except the infants, because they were incapable of making their own choice.”