What is interphase why isn't interphase part of mitosis?

Interphase is that portion of the cell cycle consisting of G1, S, and G2 phases. During these phases, the cell grows in volume, synthesizes proteins and RNAs, and replicates centrioles (in gap phases G1 and G2), whereas DNA is replicated in the intervening S phase. Mitosis properly includes on those parts of the cell cycle where segregation of genetic material and division of cytoplasmic contents occurs, and since this requires the action of microtubules / spindle fibers, only prophase, metaphase, anaphase, and telophase are included in the mitotic division of the cell cycle.