Why don't Catholics believe that Christ's atonement removed Original Sin?

Original Sin is the Catholic doctrine that all human are born sinners due to the inheritance of the sin of Adam. However, Jesus Christ atoned for the sin of mankind. Therefore, one would assume the sin of Adam which Man inherited would be included, and thus children would not need to be baptised in order to be saved as they would be innocent. Why do Catholics believe original sin was not washed away along with other sins?