Why did confirmation become separate from baptism in Western Christianity?

In the early centuries of Christianity, baptism and the rite of admission to full church membership were performed together, primarily for adult converts. The separation of these rites in Western Christianity was driven by the shift toward infant baptism and the specific role of the bishop.12

As infant baptism became the standard practice, a distinction between the two rites became necessary. In the West, the laying on of hands and the consecration of chrism were reserved for the episcopate. As dioceses grew in size, it became increasingly difficult for a bishop to be present at every baptism, causing confirmation to evolve into a separate event.12

While the rites were often performed together in the East, they became permanently separated in the West by the 16th century. Over time, confirmation in Western tradition became associated with the age of reason, serving as an occasion for formal instruction in the faith and the strengthening of the grace initially bestowed at baptism.21