Although the process of chromosome partitioning during mitosis is visible through a light microscope, the process of DNA replication is not. Why?

Because chromosomes are highly condensed during mitosis

Explanation:

Chromosomes are structures of packed and organized DNA molecules together with proteins. They are only visible under a light microscope only when the cell is undergoing the metaphase of cell division (aligned in the center-metaphase plate). In this phase chromosomes are in their condensed form and easiest to distinguish and study.

DNA replication occurs during the S phase of interphase, which is the phase before cell division.