Why do some churches baptize babies?

Because around the 3rd century they began making the terrible mistake of assuming that baptism saves. Many babies died in infancy and early childhood in those days, and since original sin is a Bible doctrine, they had to baptize babies to comfort mothers that their dying babies were saved, so they started baptizing babies. Those who could not accept infant baptism, invented the age of accountability to get all their babies into heaven. Both infant baptism and an age of accountability are heresies – beliefs that are contrary to the Bible.

For further study about baptism:

Catechism about baptism: Catechism.
All the Bible teaches about baptism: Baptism.