Why must each chromosome make a copy of itself before the m phase begins

The each chromosome makes the copy of itself before the M Phase in mitosis because the resultant daughter cell the chromosomes is genetically identical.

Explanation:

The parent cell in the chromosomes make the copy before mitosis. The copy is occurred during the S phase of inter phase. During the mitotic M phase, the cell divides the copied DNA and cytoplasm to make two new cells.

M phase involves two distinct related processes which is mitosis and cytokineses; the cell growth steps into the new stage and cellular energy is developed on the division in the phase.